No. 169-1 basic electricity by VAN VALKENBURGH, NOOGER & NEVILLE, INC. VOL. 1 WHERE ELECTRICITY COMES FROM ELECTRICITY IN ACTION CURRENT FLOW, VOLTAGE, RESISTANCE MAGNETISM, DC METERS a RIDER publication $2.25 basic electricity by VAN VALKENBIIRGH, NOOGER 6l NEVILLE, INC. VOL. 1 JOHN F. RIDER PUBLISHER, INC. 116 West 14th Street • New York 11, N. Y. First Edition Copyright 1954 by VAN VALKENBURGH, NOOGER AND NEVILLE, INC. All Rights Reserved under International and Pan American Conventions. This book or parts thereof may not be reproduced in any form or in any language without permission of the copyright owner. Library of Congress Catalog Card No. 54-12946 Printed in the United States of America PREFACE The texts of the entire Basic Electricity and Basic Electronics courses, as currently taught at Navy specialty schools, have now been released by the Navy for civilian use. This educational program has been an unqualified success. Since April, 1953, when it was first installed, over 25,000 Navy trainees have benefited by this instruc- tion and the results have been outstanding. The unique simplification of an ordinarily complex subject, the exceptional clarity of illustrations and text, and the plan of pre- senting one basic concept at a time, without involving complicated mathematics, all combine in making this course a better and quicker way to teach and learn basic electricity and electronics. The Basic Electronics portion of this course will be available as a separate series of volumes. In releasing this material to the general public, the Navy hopes to provide the means for creating a nation-wide pool of pre-trained technicians, upon whom the Armed Forces could call in time of national emergency, without the need for precious weeks and months of schooling. Perhaps of greater importance is the Navy’s hope that through the release of this course, a direct contribution will be made toward increasing the technical knowledge of men and women throughout the country, as a step in making and keeping America strong. Van Valkenburgh, TSooger and Neville , Inc. New York, N. Y. October, 1954 iii TABLE OF CONTENTS Vol. 1 — Basic Electricity What Electricity Is 1-1 How Electricity Is Produced 1-9 How Friction Produces Electricity 1-11 How Pressure Produces Electricity 1-19 How Heat Produces Electricity 1-20 How Light Produces Electricity . . . . 1-21 How Chemical Action Produces Electricity — Primary Cells . . . 1-23 How Chemical Action Produces Electricity — Secondary Cells . . 1-27 How Magnetism Produces Electricity 1-30 Current Flow — What It Is 1-42 Magnetic Fields 1-51 How Current Is Measured 1-60 How A Meter Works 1-74 What Causes Current Flow — EMF 1-83 How Voltage Is Measured 1-88 What Controls Current Flow — Resistance 1-98 iv WHAT ELECTRICITY IS The Electron Theory All the effects of electricity can be explained and predicted by assuming the existence of a tiny particle called the "electron." Using this "electron theory," scientists have been able to make predictions and discoveries which seemed impossible only a few years ago. The electron theory not only is the basis of design for all electrical and electronic equipment, it explains chemical action and allows chemists to predict and make new chemicals, such as the synthetic "wonder drugs." Since assuming that the electron exists has led to so many important dis- coveries in electricity, electronics, chemistry and atomic physics, we can safely assume that the electron really exists. All electrical and electronic equipment has been designed using the electron theory. Since the electron theory has always worked for everyone, it will always work for you. Your entire study of electricity will be based upon the electron theory. The electron theory assumes that all electrical and electronic effects are due to the movement of electrons from place to place or that there are too many or too few electrons in a particular place. « 1-1 WHAT ELECTRICITY IS The Electron Theory (continued) You have heard that electricity is the action of electrons in moving from point to point, or the excess or lack of electrons in a material. Before working with electricity, you will want to know exactly what an electron is and what causes it to move in a material. In order for electrons to move, some form of energy must be converted into electricity. Six forms of en- ergy can be used and each may be considered to be a separate source of electricity. However, before studying the kinds of energy which can cause an electron to move, you first must find out what an electron is. Because the electron is one part of an atom, you will need to know something about the atomic structure of matter. THE ELECTRON IS ELECTRICITY WHAT ELECTRICITY IS The Breakdown of Matter You have heard that electrons are tiny particles of electricity, but you may not have a very clear idea of the part electrons play in making up all the materials around us. You can find out about the electron by carefully ex- amining the composition of any ordinary material— say a drop of water. If you take this drop of water and divide it into two drops, divide one of these two drops into two smaller drops and repeat this process a few thou- sand times, you will have a very tiny drop of water. This tiny drop will be so small that you will need the best microscope made today in order to see it. WHAT ELECTRICITY IS The Breakdown of Matter (continued) Now if you take this tiny drop of water and try to divide it in half any further, you will not be able to see it in your microscope. Imagine that you have available a super microscope which will magnify many times as much as any microscope presently existing. This microscope can give you any magnification you want, so you can put your tiny drop of water under it and proceed to divide it into smaller and smaller droplets. As the droplet of water is divided into smaller and smaller droplets, these tiny droplets will still have all the chemical characteristics of water. However, you eventually will have a droplet so small that any further division will cause it to lose the chemical characteristics of water. This last bit of water is called a "molecule." If you examine the water molecule under high magnification, you will see that it is composed of three parts closely bonded together. 1-4 WHAT ELECTRICITY IS The Structure of the Molecule When you increase the magnifying power of the microscope, you will see that the water molecule is made up of two tiny structures that are the same and a larger structure that is different from the two. These tiny structures are called "atoms." The two tiny atoms which are the same are hydrogen atoms and the larger different one is an oxygen atom. When two atoms of hydrogen combine with one atom of oxygen, you have a molecule of water. THE WATER MOLECULE 1-5 WHAT ELECTRICITY IS The Structure of the Molecule (continued) While water is made up of only two kinds of atoms— oxygen and hydrogen — the molecules of many materials are more complex in structure. Cellu- lose molecules, the basic molecules of which wood is made, consist of three different kinds of atoms— carbon, hydrogen and oxygen. All materials are made up of different combinations of atoms to form molecules of the materials. There are only about 100 different kinds of atoms and these are known as elements: oxygen, carbon, hydrogen, iron, gold, nitrogen are all elements. The human body with all its complex tissues, bones, teeth, etc. is made up of only 15 elements, and only six of these are found in reason- able quantities. 1-6 WHAT ELECTRICITY IS The Breakdown of the Atom Now that you know that all materials are made up of molecules which con- sist of various combinations of about only 100 different types of atoms, you will want to know what all this has to do with electricity. Increase the magnification of your imaginery super microscope still further and exam- ine one of the atoms you find in the water molecule. Pick out the smallest atom you can see — the hydrogen atom — and examine it closely. You see that the hydrogen atom is like a sun with one planet spinning around it. The planet is known as an "electron" and the sun is known as the "nucleus." The electron has a negative charge of electricity and the nucleus has a positive charge of electricity. In an atom, the total number of negatively- charged electrons circling around the nucleus exactly equals the number of extra positive charges in the nucleus. The positive charges are called "protons." Besides the pro- tons, the nucleus also contains electrically neutral particles called "neu- trons," which are made up of a proton and an electron bonded together. Atoms of different elements contain different numbers of neutrons within the nucleus, but the number of electrons spinning about the nucleus always equals the number of free protons within the nucleus. Electrons in the outer orbits of an atom are attracted to the nucleus by less force than electrons whose orbits are near the nucleus. These outer electrons are called "free" electrons and may be easily forced from their orbits, while electrons in the inner orbits are called "bound" electrons since they cannot be forced out of their orbits easily. It is the motion of the free electrons that makes up an electric current. 1-7 WHAT ELECTRICITY IS Review of Electricity— What It Is Now let's stop and review what you have found out about electricity and the electron theory. Then you will be ready to study where elec- tricity comes from. 1. MOLECULE — The combination of two or more atoms. 2. ATOM — The smallest physical particle into which an element can be divided. 3. NUCLEUS — The heavy positively-charged part of the atom which does not move. 4. NEUTRON — The heavy neutral particle in the nucleus consisting of a proton and an electron. 5. PROTON — The heavy positively-charged particle in the nucleus. 6. ELECTRON — The very small negatively - charged particle which is practically weightless and circles the nucleus. 7. BOUND ELECTRONS — Electrons in the inner orbits of an atom, which cannot easily be forced out of their orbits. 8. FREE ELECTRONS — Electrons in the outer orbits of an atom, which can easily be forced out of their orbits. 9. ELECTRICITY — The effect of electrons in moving from point to point, or the effect of too many (excess) or too few (lack of) electrons in a material. 1-8 HOW ELECTRICITY IS PRODUCED The Six Sources of Electricity To produce electricity, some form of energy must be used to bring about the action of electrons. The six basic sources of energy which can be used are FRICTION, PRESSURE, HEAT, LIGHT, MAGNETISM and CHEM- ICAL ACTION. Before getting into the study of these sources, you will first find out about electric charges. 1-9 HOW ELECTRICITY IS PRODUCED Electric Charges You found that electrons travel around the nucleus of an atom and are held in their orbits by the attraction of the positive charge in the nucleus. If you could somehow force an electron out of its orbit, then the electron's action would become what is known as electricity. Electrons which are forced out of their orbits in some way will leave a lack of electrons in the material which they leave and will cause an ex- cess of electrons to exist at the point where they come to rest. This ex- cess of electrons in one material is called a "negative" charge while the lack of electrons in the other material is called a "positive" charge. When these charges exist you have what is called "static" electricity. To cause either a "positive" or "negative" charge, the electron must be moved while the positive charges in the nucleus do not move. Any material which has a "positive charge" will have its normal number of positive charges in the nucleus but will have electrons missing or lack- ing. However, a material which is negatively charged actually has an excess of electrons. You are now ready to find out how friction can produce this excess or lack of electrons to cause static electricity. 'Tteyative Change EXCESS OF ELECTRONS Positive d/kwupe LACK OF ELECTRONS 1-10 HOW FRICTION PRODUCES ELECTRICITY Static Charges from Friction You have studied the electron and the meaning of positive and negative charges, so that you are now ready to find out how these charges are pro- duced. The main source of static electricity which you will use is friction. If you should rub two different materials together, electrons may be forced out of their orbits in one material and captured in the other. The material which captures electrons would then have a negative charge and the ma- terial which loses electrons would have a positive charge. When two materials rub together, due to friction contact, some electron orbits of the materials cross each other and one material may give up electrons to the other. If this happens, static charges are built up in the two materials, and friction has thus been a source of electricity. The charge which you might cause to exist could be either positive or negative depending on which material gives up electrons more freely. Some materials which easily build up static electricity are glass, amber, hard rubber, waxes, flannel, silk, rayon and nylon. When hard rubber is rubbed with fur, the fur loses electrons to the rod— the rod becomes neg- atively charged and the fur positively charged. When glass is rubbed with silk, the glass rod loses electrons — the rod becomes positively charged and the silk negatively charged. You will find out that a static charge may transfer from one material to another without friction, but the original source of these static charges is friction. HOW FRICTION PRODUCES ELECTRICITY Attraction and Repulsion of Charges When materials are charged with static electricity they behave in a manner different from normal. For instance, if you place a positively charged ball near one which is charged negatively, the balls will attract each other. If the charges are great enough and the balls are light and free to move, they will come into contact. Whether they are free to move or not, a force of attraction always exists between unlike charges. This attraction takes place because the excess electrons of a negative charge are trying to find a place where extra electrons are needed. If you bring two materials of opposite charges together, the excess electrons of the negative charge will transfer to the material having a lack of electrons. This transfer or crossing over of electrons from a negative to a positive charge is called "discharge." Using two balls with the same type of charge, either positive or negative, you would find that they repel each other. ! HOW FRICTION PRODUCES ELECTRICITY Transfer of Static Charges through Contact While most static charges are due to friction, you will find that they may also be caused by other means. If an object has a static charge, it will in- fluence all other nearby objects. This influence may be exerted through contact or induction. Positive charges mean a lack of electrons and always attract electrons, while negative charges mean an excess of electrons and always repel electrons. If you should touch a positively charged rod to an uncharged metal bar, it will attract electrons in the bar to the point of contact. Some of these electrons will leave the bar and enter the rod, causing the bar to become positively charged and decreasing the positive charge of the rod. 'Positive (tyanye POSITIVELY CHARGED ROD ALMOST TOUCHING UNCHARGED BAR 1-13 HOW FRICTION PRODUCES ELECTRICITY Transfer of Static Charges through Contact (continued) By touching a negatively charged rod to the uncharged bar, you would cause the bar also to become negatively charged. As the negatively charged rod is brought near the uncharged bar, electrons in that portion of the bar nearest the rod would be repelled toward the side opposite the rod. The portion of the bar near the rod will then be charged positively and the opposite side will be charged negatively. As the rod is touched to the bar, some of the excess electrons in the negatively charged rod will flow into the bar to neutralize the positive charge in that portion of the bar but the opposite side of the bar retains its negative charge. When the rod is lifted away from the bar, the negative charge remains in the bar and the rod is still negatively charged but has very few excess electrons. When a charged object touches an uncharged object, it loses some of its charge to the uncharged object until each has the same amount of charge. {fautfy a San a Iterative (tyanqe NEGATIVELY CHARGED ROD ALMOST TOUCHING ■aWh-TTT-, ROD IS LESS NEGATIVELY CHARGED WHEN ROD TOUCHES BAR, ELECTRONS JOIN POSITIVE CHARGES METAL BAR NOW HAS AN EXCESS OF NEGATIVE CHARGES 1-14 HOW FRICTION PRODUCES ELECTRICITY Transfer of Static Charges through Induction You have seen what happens when you touch a metal bar with a positively charged rod. Some of the charge on the rod is transferred and the bar be- comes charged. Suppose that instead of touching the bar with the rod, you only bring the positively charged rod near to the bar. In that case, elec- trons in the bar would be attracted to the point nearest the rod, causing a negative charge at that point. The opposite side of the bar would again lack electrons and be charged positive. Three charges would then exist, the positive charge in the rod, the negative charge in the bar at the point nearest the rod and a positive charge in the bar on the side opposite the rod. By allowing electrons from an outside source (your finger, for in- stance) to enter the positive end of the bar, you can give the bar a nega- tive charge. ELECTRONS ARE ATTRACTED TOWARD CHARGED ROD lOOOOO ELECTRONS ARE ATTRACTED OFF FINGER AND ENTER BAR. lloooooj O 90 90 00 90 e 9 00 90 00 00 9 e 9 90 90 90 90 FINGER IS REMOVED. POSITIVE AND NEGATIVE ^ CHARGES ARE MOSTLY NEUTRALIZED. ROD IS REMOVED AND EXCESS ELECTRONS REMAIN e 00 90 00 00 9 9 90 00 00 90 9 a an an an a L5 HOW FRICTION PRODUCES ELECTRICITY Transfer of Static Charges through Induction (continued) If the rod is negatively charged when brought near to the bar, it will in- duce a positive charge into that end of the bar which is near the rod. Electrons in that portion of the rod will be repelled and will move to the opposite end of the bar. The original negative charge of the rod then causes two additional charges, one positive and one negative, in the bar. Removing the rod will leave the bar uncharged since the excess electrons in the negatively charged end will flow back to neutralize the bar. How- ever, if before the rod is moved a path is provided for the electrons in the negatively charged portion of the bar to flow out of the bar, the entire bar will be positively charged when the rod is removed. tfivuty a San a Positive (tyanye, ELECTRONS ARE REPELLED FROM CHARGED ROD 0 © 0© 0© 0 00 00 90 ft © © 0© CO © FINGER IS REMOVED, SOME ELECTRONS HAVE LEFT NEGATIVE END ROD IS REMOVED, BAR NOW LACKS ELECTRONS AND IS POSITIVELY CHARGED You have discovered that static charges can be caused by friction and con- tact, or induction. Now you should see how the excess or lack of electrons in the charged body may be neutralized, or discharged. 1-16 HOW FRICTION PRODUCES ELECTRICITY Discharge of Static Charge^ Whenever two materials are charged with opposite charges and placed near one another, the excess electrons on the negatively charged material will be pulled toward the positively charged material. By connecting a wire from one material to the other, you would provide a path for the electrons of the negative charge to cross over to the positive charge, and the charges would thereby neutralize. Instead of connecting the materials with a wire, you might touch them together (contact) and again the charges would disappear. If you use materials with strong charges, the electrons may jump from the negative charge to the positive charge before the two materials are actually in contact. In that case , you would actually see the discharge in the form of an arc. With very strong charges, static electricity can dis- charge across large gaps, causing arcs many feet in length. Lightning is an example of the discharge of static electricity resulting from the accumulation of a static charge in a cloud as it moves through the air. Natural static charges are built up wherever friction occurs be- tween the air molecules, such as with moving clouds or high winds, and you will find that these charges are greatest in a very dry climate, or elsewhere when the humidity is low. 1-17 HOW FRICTION PRODUCES ELECTRICITY Review of Friction and Static Electric Charges You have now found out about friction as a source of electricity, and you have seen and participated in a demonstration of how static electric charges are produced and their, effect on charged and uncharged materials. You have also seen how static charges can be transferred by contact or in- duction, and you have learned about some of the useful applications of static electricity. Before going on to learn about the other basic sources of electricity, you should review those facts which you have already learned. 1 . NEGATIVE CHARGE — An excess of electrons. 2. POSITIVE CHARGE — A lack of electrons. 3. REPULSION OF CHARGES — Like charges repel each other. 4. ATTRACTION OF CHARGES — Unlike charges attract each other. 5. STATIC ELECTRICITY — Electric charges at rest. 6. FRICTION CHARGE — A charge caused by rubbing one material' against another. CONTACT CHARGE — Transfer of a charge from one material to another by direct contact. INDUCTION CHARGE — Transfer of a charge from one material to another without actual contact. CONTACT DISCHARGE — Electrons crossing over from a negative charge to positive through contact. ARC DISCHARGE — Electrons crossing over from a negative charge to positive through an arc. When you have completed your review of friction and static electric charges, you will go on to learn about pressure as a source of electricity. 1-18 HOW PRESSURE PRODUCES ELECTRICITY Electric Charges from Pressure Whenever you speak into a telephone, or other type of microphone, the pressure waves of the sound energy move a diaphragm. In some cases, the diaphragm moves a coil of wire past a magnet, generating electrical energy which is transmitted through wires to a receiver. Microphones used with public address systems and radio transmitters sometimes oper- ate on this principle. Other microphones, however, convert the pressure waves of sound directly into electricity. Crystals of certain materials will develop electrical charges if a pressure is exerted on them. Quartz, tourmaline, and Rochelle salts are materials which illustrate the principle of pressure as a source of electricity. If a crystal made of these materials is placed between two metal plates and a pressure is exerted on the plates, an electric charge will be developed. The size of the charge produced between the plates will depend on the amount of pressure exerted. The crystal can be used to convert electrical energy to mechanical energy by placing a charge on the plates, since the crystal will expand or con- tract depending on the amount and type of the charge. While the actual use of pressure as a source of electricity is limited to very low power applications, you will find it in many different kinds of equipment. Crystal microphones, crystal headphones, phonograph pickups and sonar equipment use crystals to generate electric charges from pressure. 1-19 HOW HEAT PRODUCES ELECTRICITY Electric Charges from Heat Another method of obtaining electricity is to convert heat into electricity directly, by heating a junction of two dissimilar metals. For example, if an iron wire and a copper wire are twisted together to form a junction, and the junction is heated, an electric charge will result. The amount of charge produced depends on the difference in temperature between the junction and the opposite ends of the two wires. A greater temperature difference results in a greater charge. A junction of this type is called a thermo-couple and will produce elec- tricity as long as heat is applied. While twisted wires may form a thermo- couple, more efficient thermo-couples are constructed of two pieces of dissimilar metal riveted or welded together. Thermo-couples do not furnish a large amount of charge and cannot be used to obtain electric power. They are normally used in connection with heat indicating devices to operate a meter directly marked in de- grees of temperature. 1-20 HOW LIGHT PRODUCES ELECTRICITY Electric Charges from Light — Photovoltaic Effects Electricity may be produced by using light as the source of energy con- verted to electricity. When light strikes certain materials, they may con- duct electric charges easier, develop an electric charge, emit free elec- trons or convert light to heat. The most useful of these effects is the development of an electric charge by a photo cell when light strikes the photo-sensitive material in the cell. A photo cell is a metallic "sandwich" or disc composed of three layers of material. One outside layer is made of iron. The other outside layer is a film of translucent or semitransparent material which permits light to pass through. The center layer of material is composed of selenium alloy. The two outside layers act like electrodes. When light is focused on the selenium alloy through the translucent material an electric charge is de- veloped between the two outside layers. If a meter is attached across these layers the amount of charge can be measured. A direct use of this type of cell is the common light meter as used in photography for deter- mining the amount of light which is present. 1-21 HOW LIGHT PRODUCES ELECTRICITY Electric Charges from Light — Photo Electric Cell or Phototube The photo electric cell, commonly called an "electric eye" or a "PE Cell," operates on the principle of the photo cell. The photo electric cell, how- ever, depends upon a battery or some other source of electrical pressure in its operation of detecting changes in light. The photo cell has many uses, some of which are automatic headlight dimmers on automobiles, mo- tion picture machines, automatic door openers and drinking fountains. Photo Electric Cell with light not burning 1-22 HOW CHEMICAL ACTION PRODUCES ELECTRICITY— PRIMARY CELLS Electricity from Chemical Action So far, you have discovered what electricity is and several sources of en- ergy which may be used to produce it. Another source of electricity com- monly used is the chemical action housed in electric cells and batteries. Batteries are usually used for emergency and portable electric power. Whenever you use a flashlight emergency lantern or portable equipment, you will be using batteries. Batteries are the main source of power for present-day submarines. In addition, there is a wide variety of equipment which uses cells or batteries either as normal or emergency power. "Dead" batteries are a common type of equipment failure and such failures can be very serious. Cells and batteries require more care and maintenance than most of the equipment on which you will work. Even though you may use only a few cells or batteries, if you find out how they work, where they are used and how to properly care for them, you will save time and in many cases a lot of hard work. Now you will find out how chemical action produces electricity and the proper use and care of the cells and batteries that house this chemi- cal action. 1-23 HOW CHEMICAL ACTION PRODUCES ELECTRICITY— PRIMARY CELLS A Primary Cell — What It Is To find Out how the chemical action in batteries works, you might imagine that you can see electrons and what happens to them in a primary elec- tric cell. The basic source of electricity produced by chemical action is the electric cell and, when two or more cells are combined, they form a battery. Now if you could see the inner workings of one of these cells, what do you suppose you would see? First you would notice the parts of the cell and their relation to each other. You would see a case or container in which two plates of different me- tals, separated from each other, are immersed in the liquid which fills the container. Watching the parts of the cell and the electrons in the cell you would see that the liquid which is called the electrolyte is pushing electrons onto one of the plates and taking them off the other plate. This action results in an excess of electrons or a negative charge on one of the plates so that a wire attached to that plate is called the negative terminal. The other plate loses electrons and becomes positively charged so that a wire at- tached to it is called the positive terminal. The action of the electrolyte in carrying electrons from one plate to the other is actually a chemical reaction between the electrolyte and the two plates. This action changes chemical energy into electrical charges on the cell plates and terminals. 1-24 HOW CHEMICAL ACTION PRODUCES ELECTRICITY— PRIMARY CELLS Chemical Action in a Primary Cell With nothing connected to the cell terminals, you would see that electrons are pushed onto the negative plate until there is room for no more. The electrolyte would take from the positive plate enough electrons to make up for those it pushed onto the negative plate. Both plates would then be fuUy charged and no electrons would be moving between the plates. Now suppose you connected a wire between the negative and positive ter- minals of the cell. You would see the electrons on the negative terminal leave the terminal and travel through the wire to the positive terminal. Since there would now be more room on the negative terminal, the electro - lyte would carry more electrons across from the positive plate to the neg- ative plate. As long as electrons leave the negative terminal and travel to the positive terminal outside the cell, the electrolyte will carry electrons from the positive plate to the negative plate inside the cell. While the electrolyte is carrying electrons, you would see that the nega- tive plate is being used up and you would notice bubbles of gas at the posi- tive terminal. Eventually the negative plate would be completely dissolved in the electrolyte by the chemical action, and the cell would be "dead," or unable to furnish a charge, until the negative plate is replaced. For that reason, this type of cell is called a primary cell — meaning that once it has completely discharged, it cannot be charged again except by using new materials. For plates in a primary cell, carbon and most metals can be used, while acids or salt compounds can be used for the electrolyte. Dry cells such as those used in flashlights and lanterns are primary cells. 1-25 HOW CHEMICAL ACTION PRODUCES ELECTRICITY— PRIMARY CELLS Dry Cells and Batteries Almost any metals, acids and salts can be used in primary cells. There are many types of primary cells used in laboratories and for special ap- plications, but the one which you have used and will be using most often is the dry cell. You will use the dry cell in many different sizes, shapes and weights — from the cell used in a pencil-size flashlight to the extra large dry cell used in emergency lanterns. Regardless of size, you will find that the material used and the operation of all dry cells is the same. If you were to look inside a dry cell, you would find that it consists of a zinc case used as the negative plate, a carbon rod suspended in the center of the case for the positive plate, and a solution of ammonium chloride in paste form for the electrolyte. At the bottom of the zinc case you would see a tar paper washer used to keep the carbon rod from touching the zinc case. At the top, the casing would contain layers of sawdust, sand and pitch. These layers hold the carbon rod in position and prevent electro- lyte leakage. When a dry cell supplies electricity, the zinc case and the electrolyte are gradually used up. After the usable zinc and electrolyte are gone, the cell cannot supply a charge and must be replaced. Cells of this type are sealed and can be stored for a limited time without causing damage. When several such cells are connected together, they are called a dry battery. You cannot use dry cells to furnish large amounts of power so you will find them only where infrequent and emergency use is intended. 1-26 HOW CHEMICAL ACTION PRODUCES ELECTRICITY —SECONDARY CELLS A Secondary Cell — What It Is In studying primary cells, you learned that chemical action is com- monly used as a source of electric power for emergency or portable equipment. However, it will furnish only a small amount of power and cannot be recharged. A storage battery of secondary cells can furnish large for a short time and can be recharged. Batteries of this type require more maintenance and care than dry cell batteries but a ^ e us ®^ ^ equipment where large amounts of electricity are needed for short periods of time. Secondary cells used in storage batteries are of the lead-acid type. In this cell the electrolyte is sulphuric acid while the positive plate is lead peroxide and the negative plate is lead. During discharge of the cell, the acid becomes weaker and both plates change chemically to lead sulfate. The case of a lead-acid cell is made of hard rubber or glass, which pre- vents corrosion and acid leaks. A space at the bottom of the cell collects the sediment formed as the cell is used. The top of the case is removable and acts as the support for the plates. + I *“ Electrolyte Cell Plates Case Cover Separator Sediment Collector LEAD-ACID SECONDARY CELL 1-27 HOW CHEMICAL ACTION PRODUCES ELECTRICITY —SECONDARY CELLS A Secondary Cell— What It Is (continued) Since the active materials are not rigid enough to be mounted independ- ently, a special grid structure of inactive metal is used to hold them. For maximum chemical action, a large plate area is desired, so each positive plate is interlaced between two negative plates. In a typical cell, you might find seven positive plates attached to a common support interlaced with eight negative plates attached to a different support. Separators, made of wood or porous glass, hold each positive and negative plate apart but let the electrolyte pass through. The positive and negative plates are fastened to the case cover which is held in place by a special acid-resistant tar. An opening in the cover allows water to be added to the electrolyte to replace water which evapo- rates. The cap for this opening has a vent to allow gas to escape since the cell in operation forms gas at the positive plate. Since these cells furnish large amounts of electricity, they require larger terminals and leads. Connections and terminals are made of lead bars since other metals would corrode rapidly due to the acid electrolyte. Case 1-28 SINGLE CELL COMPONENTS HOW CHEMICAL ACTION PRODUCES ELECTRICITY — SECONDARY CELLS Storage Batteries When two or more secondary cells are connected together, they form a storage battery. This battery stores electricity and can be recharged after discharge. Most storage batteries consist of three lead-acid cells in a common case permanently connected in series. Since each lead-acid cell is rated at about two volts, connecting three cells in series produces a battery volt- age of six volts. The symbol for a secondary cell is the same as that used for a primary cell and the storage battery symbol shows three cells connected in series. Storage batteries and secondary cells are not connected in parallel since a weaker cell would cause a stronger cell to discharge, thus lowering battery strength without the battery even being used. 1-29 HOW MAGNETISM PRODUCES ELECTRICITY Electric Power from Magnetism The most common method of producing electricity used for electric power is by the use of magnetism. The source of electricity must be able to maintain a large charge because the charge is being used to furnish elec- tric power. While friction, pressure,. heat and light are sources of elec- tricity, you have found that their use is limited to special applications since they are not capable of maintaining a large enough charge for electric power. All of the electric power used, except for emergency and portable equip- ment, originally comes from a generator in a power plant. The generator may be driven by water power, a steam turbine or an internal combustion engine. No matter how the generator is driven, the electric power it pro- duces is the result of the action between the wires and the magnets inside the generator. When wires move past a magnet or a magnet moves past wires, electric- ity is produced in the wires because of the magnetism in the magnetic material. Now you will find out what magnetism is and how it can be used 1-30 HOW MAGNETISM PRODUCES ELECTRICITY Magnetism — What It Is In ancient times, the Greeks discovered that a certain kind of rock, which they originally found near the city of Magnesia in Asia Minor, had the power to attract and pick up bits of iron. The rock which they discovered was actually a type of iron ore called "magnetite," and its power of at- traction is called "magnetism." Rocks containing ore which has this power of attraction, are called natural magnets. hatural " MA6NET Natural magnets were little used until it was discovered that a magnet mounted so that it could turn freely would always turn so that one side would point to the north. Bits of magnetite suspended on a string were called "lodestones," meaning a leading stone, and were used as crude compasses for desert travel by the Chinese more than 2,000 years ago. Crude mariner's compasses constructed of natural magnets were used by sailors in the early voyages of exploration. The earth itself is a large natural magnet and the action of a natural mag- net in turning toward the north is caused by the magnetism or force of attraction of the earth. 1-31 HOW MAGNETISM PRODUCES ELECTRICITY Magnetism— What It Is (continued) In using natural magnets, it was found that a piece of iron stroked with a natural magnet became magnetized to forman artificial magnet. Artificial magnets may also be made electrically and materials other than iron may be used to form stronger magnets. Steel alloys containing nickel and cobalt make the best magnets and are usually used in strong magnets. Iron becomes magnetized more easily than other materials but it also loses its magnetism easily so that magnets of soft iron are called tempo- rary magnets. Magnets made of steel alloys hold their magnetism for a long period of time and are called permanent magnets. Magnetism in a magnet is concentrated at two points, usually at the ends of the magnet. These points are called the "poles" of the magnet — one being called the "north pole," the other the "south pole." The north pole is at the end of the magnet which would point north if the magnet could swing freely, and the south pole is at the opposite end. Magnets are made in various shapes, sizes and strengths. Permanent magnets are usually made of a bar of steel alloy, either straight with poles at the ends, or bent in the shape of the familiar horseshoe with poles on opposite sides of the opening. 1-32 HOW MAGNETISM PRODUCES ELECTRICITY Magnetism— What It Is (continued) Magnetism is an invisible force and can be seen only in terms of the effect it produces. You know that the wind, for example, provides tremendous force, yet it is invisible. Similarly, magnetic force may be felt but not seen. The magnetic field about a magnet can best be explained as invisible lines of force leaving the magnet at one point and entering it at another. These invisible lines of force are referred to as ’’flux lines" and the shape of the area they occupy is called the "flux pattern." The number of flux lines per square inch is called the "flux density." The points at which the flux lines leave or enter the magnet are called the "poles." The magnetic circuit is the path taken by the magnetic lines of force. If you were to bring two magnets together with the north poles facing each other, you would feel a force of repulsion between the poles. Bringing the south poles together would also result in repulsion but, if a north pole is brought near a south pole, a force of attraction exists. In this respect, mag- netic poles are very much like static charges. Like charges or poles repel each other and unlike charges or poles attract. The action of the magnetic poles in attracting and repelling each other is due to the magnetic field around the magnet. As has already been ex- plained, the invisible magnetic field is represented by lines of force which leave a magnet at the north pole and enter it at the south pole. Inside the magnet the lines travel from the south pole to the north pole so that a line of force is continuous and unbroken. 1-33 HOW MAGNETISM PRODUCES ELECTRICITY Magnetism — What It Is (continued) One characteristic of magnetic lines of force is such that they repel each other, never crossing or uniting. If two magnetic fields are placed near each other, as illustrated by the placement of the two magnets, below, the magnetic fields will not combine but will reform in a distorted flux pattern, (Note that the flux lines do not cross each other. ) An Example of Bypassing Flux Lines There is no known insulator for magnetic lines. It has been found that flux lines will pass through all materials. However, they will go through some materials more easily than others. This fact makes it possible to concen- trate flux lines where they are used or to bypass them around an area or instrument. MAGNETIC SCREEN 1-34 HOW MAGNETISM PRODUCES ELECTRICITY Magnetism — What It Is (continued) On the previous sheet you were told that magnetic lines of force will go through some materials more easily than through others. Those materials which will not pass flux lines so readily, or which seem to hinder the passage of the lines, are said to have a comparatively high "reluctance" to magnetic fields. Materials which pass, or do not hinder the "flow" of flux lines, are said to have a comparatively low reluctance to magnetic fields of force. Reluctance, with reference to a magnetic circuit, is roughly the equivalent of resistance, when an electric circuit is considered. Magnetic lines of force take the path of least reluctance; for example, they travel more easily through iron than through the air. Since air has a greater reluctance than iron, the concentration of the magnetic field be- comes greater in the iron (as compared to air) because the reluctance is decreased. In other words, the addition of iron to a magnetic circuit con- centrates the magnetic field which is in use. Magnetic lines of force act like stretched rubber bands. The figure below suggests why this characteristic exists, particularly near the air gap. Note that some lines of force curve outward across the gap in moving from the north pole to the south pole. This outward curve, or stretching effect, is caused by the repulsion of each magnetic line from its neighbor. How- ever, the lines of force tend to resist the stretching effect and therefore resemble rubber bands under tension. Unlike poles attract 1-35 HOW MAGNETISM PRODUCES ELECTRICITY Magnetism — What It Is (continued) As has already been mentioned, magnetic lines of force tend to repel each other. By tracing the flux pattern of the two magnets with like poles to- gether, in the diagram below, it can be seen why this characteristic exists. The reaction between the fields of the two magnets is caused by the fact that lines of force cannot cross each other. The lines, therefore, turn aside and travel in the same direction between the pole faces of the two magnets. Since lines of force which are moving in such a manner tend to push each other apart, the magnets mutually repel each other. Only a certain number of magnetic lines can be crowded into a piece of material. This varies with each type of material but when the maximum number has been attained the material is said to be saturated. This phenomenon is made use of in many pieces of electrical equipment. The property of magnetism may be induced, or introduced, in a piece of material which does not ordinarily have that characteristic. If a piece of unmagnetized soft iron is placed in the magnetic field of a permanent mag- net the soft iron assumes the properties of a magnet; it becomes magnet- ized. This action, or process, is called magnetic induction and arises from the fact that magnetic lines of force tend to flow through a material which offers less reluctance than air to their passage. When the lines of the magnetic field pass through the soft iron bar (see the diagram below), the molecules of the soft iron line up parallel with the lines of force, and with their north poles pointing in the direction that the lines of force are traveling through the iron. Magnetism then, is induced in the soft iron bar and in the polarity indicated. If the permanent magnet is removed, the soft iron bar will lose a good deal of its magnetic quality. The amount of magnetism which remains is called residual magnetism. The term "residual magnetism" is encountered later in this course and in the study of DC generators. effect oe A SOFT IRC* BAR U* A 1-36 HOW MAGNETISM PRODUCES ELECTRICITY Demonstration — Magnetic Fields To show that unlike magnetic poles attract each other, the instructor brings two bar magnets near each other with the north pole of one magnet approaching the south pole of the other. Notice that the magnets not only come together easily but attract each other strongly, showing that unlike poles attract. However, when the two magnets are brought together with similar poles opposing, it is difficult to force the magnets together, indi- cating that like poles repel each other. When the demonstration is re* peated using horseshoe magnets, the results are the same like poles repel, unlike poles attract. , ATTRACTION REPULSION To show how lines of force form a magnetic field around a magnet, the in- structor will use a bar magnet, a horseshoe magnet and iron filings to trace out a pattern of the magnetic field. He places a sheet of lucite over the magnet and then he sprinkles iron filings on the lucite. Observe that the iron filings do not evenly cover the sheet of lucite. Instead they ar- range themselves in a definite pattern, with many more filings attracted to the magnet poles than to other places on the lucite. You also see that the filings arrange themselves in a series of lines around the poles, in- dicating the pattern of the magnetic lines of force which make up the magnetic field. 1-37 HOW MAGNETISM PRODUCES ELECTRICITY Movement of a Magnet Past a Wire One method by which magnetism produces electricity is through the move- ment of a magnet past a stationary wire. If you connect a very sensitive meter across the ends of a stationary wire and then move a magnet past the wire, the meter needle will deflect. This deflection indicates that electricity is produced in the wire. Repeating the movement and observ- ing the meter closely, you will see that the meter moves only while the magnet is passing near the wire. Placing the magnet near the wire and holding it at rest, you will observe no deflection of the meter. Moving the magnet from this position, however, does cause the meter to deflect and shows that, alone, the magnet and wire are not able to produce electricity. In order to deflect the needle, move- ment of the magnet past the wire is necessary. Movement is necessary because the magnetic field around a magnet pro- duces an electric current in a wire only when the magnetic field is moved across the wire. When the magnet and its field are stationary, the field is not moving across the wire and will not produce a movementof electrons 1-38 HOW MAGNETISM PRODUCES ELECTRICITY Movement of a Wire Past a Magnet In studying the effect of moving a magnet past a wire, you discovered that electricity was produced only while the magnet and its field were actually moving past the wire. If you move the wire past a stationary magnet, you again will notice a deflection of the meter. This deflection will occur only while the wire is moving across the magnetic field. To use magnetism to produce electricity, you may either move a magnetic field across a wire or move a wire across a magnetic field. For a continuous source of electricity, however, you need to maintain a continuous' motion of either the wire or the magnetic field. To provide continuous motion, the wire or the magnet would need to move back and forth continuously. A more practical way is to cause the wire to travel in a circle through the magnetic field. This method of producing electricity— that of the wire traveling in a circle past the magnets — is the principle of the electric generator and is the source of most electricity used for electric power. 1-39 HOW MAGNETISM PRODUCES ELECTRICITY Movement of a Wire Past a Magnet (continued) To increase the amount of electricity which can be produced by moving a wire past a magnet, you might increase the length of the wire that passes through the magnetic field, use a stronger magnet or move the wire faster. The length of the wire can be increased by winding it in several turns to form a coil. Moving the coil past the magnet will result in a much greater deflection of the meter than resulted with a single wire. Each additional coil turn will add an amount equal to that of one wire. ^ LLLUH .'hit// COIL OF WIRE MOVING PAST THE MAGNET I M /JJJ Moving a coil or a piece of wire past a weak magnet causes a weak flow of electrons. Moving the same coil or piece of wire at the same speed past a strong magnet will cause a stronger flow of electrons, as indicated by the meter deflection. Increasing the speed of the movement also results in a greater electron flow. In producing electric power, the output ,of an electric generator is usually controlled by changing either (1) the strength of the magnet or (2) the speed of rotation of the coil. INCREASING SPEED OF COIL OF WIRE PAST THE MAGNET / mi Hi hitmi USING A STRONGER MAGNET WHERE ELECTRICITY COMES FROM Review of Electricity and How It Is Produced To conclude your study of how electricity is produced, suppose you review briefly what you have found out about electricity and where it comes from. FLECTRICITY is the action of electrons which have been forced from Sr normal orbits around the nucleus ot an atom. To force electrons out S lhe"?Tbits. so they can become a source of electricity, some kind ot energy is required. Six kinds of energy can be used: FRICTION — Electricity produced by rub- bing two materials together. PRESSURE — Electricity produced by ap- plying pressure to a crystal of certain materials. HEAT — Electricity produced by heating the junction of a thermo-couple. LIGHT — Electricity produced by light striking photo- sensitive materials. MAGNETISM — Electricity produced by relative movement of a magnet and a wire that results in the cutting of lines of force. <-hfmtc?AL ACTION — Electricity produced by chemical reaction in an electric cell. CURRENT FLOW— WHAT IT IS Electrons in Motion Electrons in the outer orbits of an atom are attracted to the nucleus by less force than electrons whose orbits are near the nucleus. These outer elec- trons may be easily forced from their orbits, while electrons in the inner orbits are called bound" electrons since they cannot be forced out of tneir orbits. Atoms and molecules in a material are in continuous random motion, the amount of this motion determined by the material, temperature and pres- sure. This random motion causes electrons in the outer rings to be forced from their orbits, becoming "free" electrons. "Free" electrons are at- tracted to other atoms which have lost electrons, resulting in a continuous passage of electrons from atom to atom within the material. All electrical effects make use of the "free" electrons forced out of the outer orbits. The atom itself is not affected by the loss of electrons, except that it be- comes positive^ charged and will capture "free" electrons to replace those it has lost. F The random movement of the "free" electrons from atom to atom is nor- mally equal in aU directions so that electrons are not lost or gained by any particular part of the material. When most of the electron movement lakes place m the same direction, so that one part of the material loses electrons whiie another part gams electrons, the net electron movement or flow is called current flow. 1-42 CURRENT FLOW— WHAT IT IS Electrons in Motion (continued) Suppose you examine more closely what happens inside a material when an electron current begins to flow. You learned in Section II, Topic 1 that an atom is made up of a number of neutrons, protons and electrons. The pro- tons have a positive charge, the electrons a negative charge, and the neu- trons have no charge at all. The nucleus of the atom is made up of neu- trons and protons and has a positive charge equal to the number of protons. Under normal conditions the number of electrons traveling around the nu- cleus equals the number of protons in the nucleus and the entire atom has no charge at all. If an atom loses several of its free electrons, it then has a positive charge since there are more protons than electrons. From your work in Section n you know that like charges repel each other and unlike charges attract. About each positive or negative charge, unseen lines of force radiate in all directions and the area occupied by these lines is called an "electric field." Thus if a moving electron comes close to another electron, the second electron will be pushed away without the two electrons coming into contact. Similarly, if an electron comes near a positive charge, the two fields reach out and attract each other even though there may be some distance between them. It is the attraction between the positive charge on the nucleus and the electrons in the outer orbit that determine the electrical characteristics of a material. If the atom of a particular material is so constructed that there is a very small attraction between the positive nucleus and the outer electrons, the outer electrons are free to leave the atom when they are under the influence of electric fields. Such a condition exists in metals; and silver, copper and aluminum have a very weak attractive force between the nucleus and the outer electrons. Substances such as glass, plastic, wood and baked clays have a very powerful bond between the nucleus and the outer electrons, and these electrons will not leave their atoms unless very strong electrical fields are applied. 1-43 CURRENT FLOW— WHAT IT IS Electrons in Motion (continued) You learned that a dry cell has the peculiar property of having an excess of electrons at its negative terminal and a shortage of electrons at its pos- itive terminal. Suppose you examine just what happens when a metal wire is connected across the terminals of a dry cell. The moment the wire is connected across the cell there will be an excess of electrons at the negative end and a shortage at the positive end. Re- member that electrons repel each other and are attracted to places where there is a shortage of electrons. At the negative end of the cell the excess of electrons now have a place to go. The electric fields of these electrons push against the electrons in the atoms of the wire, and some of these outer electrons are pushed out of their atoms. These free electrons can- not remain where they are since their electric fields force them away from the piled up electrons at the negative terminal, so they are forced away from the negative terminal. When these newly freed electrons arrive at the next atom, they in turn force those outer electrons off their atoms and the process continues. At the positive end of the wire there is a shortage of electrons, and there- fore there is a strong attraction between the positive terminal and the outer electrons of the nearby atoms. These electric fields of the outer electrons are strongly pulled by the electric field of the positive terminal, and some of the electrons leave their atoms and move toward the positive end. When these electrons leave their atoms, the atoms become positively charged and electrons from the next atoms are attracted toward the positive end' and the process continues. ’ 1-44 CURRENT FLOW— WHAT IT IS Electrons in Motion (continued) If the excess and shortage of electrons at the two ends of the wire were fixed at a definite quantity, it would only be a very short time until all the excess electrons had traveled through the wire toward the positive end. The dry cell, however, continues to furnish excess electrons at one ter- minal and continues to remove electrons from the other terminal, so that the two terminals remain negative and positive for the life of the cell. Under these conditions a constant stream of electrons begins to flow through the wire the instant the wire is connected to the cell. Electrons constantly arriving at the negative end keep applying a pushing force to the free elec- trons in the wire, and the constant removal of electrons from the positive end keeps applying a pulling force on the free electrons. The movement of electrons through the battery and wire would look like this 1-45 CURRENT FLOW— WHAT IT IS Electrons In Motion (continued) If you have any difficulty in picturing what is happening inside the wire, suppose you examine a similar situation which makes use of more familiar components. Imagine a large piece of drain pipe in which a large number of golf balls are suspended by means of wires. Each golf ball represents an atom with its bound electrons. Now fill all of the space between the golf balls with small metal balls the size of air rifle shot (BB shot). Each small ball represents a free electron. Now imagine an army of little men removing the BB's from one end of the pipe and ramming them back into the other end. This army represents the dry cell. Since the pipe cannot be packed any more tightly, and since it is too strong to burst, all that can happen is that there will be a constant flow of small metal balls through the pipe. The faster the little men work and the harder they push, the greater will be the flow of BB shot. The flow begins at the instant the army begins to work, and continues at the same rate until the little men are too exhausted to move any more — the dry cell is then "dead." A very similar situation exists between the drain pipe and a wire carrying an electric current. The main difference is that in the pipe, the metal balls press directly upon each other, while in the wire, the electrons them- selves do not touch but their electric fields press against each other. 1-46 CURRENT FLOW— WHAT IT IS Electrons In Motion (continued) When current flow starts in a wire, electrons start to move throughout the wire at the same time, just as the cars of a long train start and stop together. If one car of a train moves, it causes all the cars of the train to move by the same amount, and free electrons in a wire act in the same manner. Free electrons are always present throughout the wire, and as each elec- tron moves slightly it exerts a force on the next electron, causing it to move slightly and in turn to exert a force on the next electron. This effect continues throughout the wire. When electrons move away from one end of a wire it becomes positively charged, causing all the free electrons in the wire to move in that direction. This movement , taking place throughout th§ wire simultaneously, moves electrons away from the other end of the wire and allows more electrons to enter the wire at that point. ELECTRONS MOVING IN A WIRE... Each electron forces the next to move slightly /7/ry,N;r ..ALL START AT THE SAME TIME When one car moves they all move 1-47 CURRENT FLOW— WHAT IT IS Electrons in Motion (continued) Since electrons repel each other and are attracted by positive charges, they always tend to move from a point having an excess of electrons toward a point having a lack of electrons. Your study of the discharge of static charges showed that, when a positive charge is connected to a negative charge, the excess electrons of the negative charge move toward the posi- tive charge. If electrons are taken out of one end of a copper wire, a positive charge results, causing the free electrons in the wire to move toward that end. If electrons are furnished to the opposite end of the wire, causing it to be charged negatively, a continuous movement of electrons will take place from the negatively charged end of the wire toward the positively charged end. This movement of electrons is current flow and will continue as long as electrons are furnished to one end of the wire and removed at the other end. Current flow can take place in any material where "free" electrons exist, although we are only interested in the current flow in metal wires. CURRENT FLOW —WHAT IT IS Direction of Current Flow According to the electron theory; current flow is always from a (-) nega- tive charge to a (+) positive charge. Thus, if a wire is connected between the terminals of a battery, current will flow from the (-) terminal to the (+) terminal. Before the electron theory of matter had been worked out, electricity was in use to operate lights, motors, etc. Electricity had been harnessed but no one knew how or why it worked. It was believed that something moved in the wire from (+) to (-). This conception of current flow is called con- ventional current flow. Although the electron theory of current flow (-) to (+) is the accepted theory, you will find the conventional flow (+) to (-) is sometimes used in working with certain types of electrical equipment. For your study of electricity, current flow is concluded to be the same as the electron flow— that is, from negative to positive. CURRENT FLOW— WHAT IT IS Review of Current Flow Current flow does all the work involved in the operation of electrical equip- ment, whether it be a simple light bulb or some complicated electronic equipment such as a radio receiver or transmitter. In order for current to flow, a continuous path must be provided between the two terminals of a source of electric charges. Now suppose you review what you have found out about current flow. 1. “BOUND" ELECTRONS — Electrons in the inner orbits of an atom which cannot easily be forced out of their orbits. 2. "FREE" ELECTRONS — Electrons in the outer orbits of an atom which can easily be forced out of their orbits. 3. CURRENT FLOW — Movement of "free" elec- trons in the same direction in a material. I. ELECTRON CURRENT — Current flow from a negative charge to a positive charge. 5. CONVENTIONAL CURRENT — Current flow from a positive charge to a negative charge. 6. AMMETER — Meter used to measure amperes. 1-50 MAGNETIC FIELDS Electromagnetism In the previous topic you learned the very important fact that an electric current can be caused to flow when you move a coil of wire so that it cuts through a magnetic field. You also learned that this is the most widespread manner in which electricity is generated for the home, for industry, and aboard ship. Since magnetism can be made to generate electricity, it does not seem too great a jump for the imagination to wonder if electricity can generate a magnetic field. In this topic you will see for yourself that that is exactly what can be done. In the last topic you made use of permanent magnets to cause an electric current to flow. You saw that more current could be generated as you in- creased the number of turns of wire, the speed of motion of the coil and the strength of the magnetic field. It is a simple matter to accomplish the first two of these in a practical electric generator, but it is very difficult to in- crease the strength of a permanent magnet beyond certain limits. In order to generate large amounts of electricity a much stronger magnetic field must be used. That is accomplished, as you will see in this topic, by means of an electromagnet. Electromagnets work on the simple principle that a magnetic field can be generated by passing an electric current through a coil of wire. 1-51 MAGNETIC FIELDS Electromagnetism (continued) An electromagnetic field is a magnetic field caused by the current flow in a wire. Whenever electric current flows, a magnetic field exists around the conductor, and the direction of this magnetic field depends upon the direction of current flow. The illustration shows conductors carrying cur- rent in different directions. The direction of the magnetic field is counter- clockwise when current flows from left to right. If the direction of current flow reverses, the direction of the magnetic field also reverses, as shown. In the cross-sectional view of the magnetic field around the conductors, the dot in the center of the circle represents the current flowing out of the paper toward you, and the cross represents the current flowing into the paper away from you. A definite relationship exists between the direction of current flow in a wire and the direction of the magnetic field around the conductor. This relationship can be shown by using the left-hand rule. This rule states that if a current-carrying conductor is grasped in the left hand with the thumb pointing in the direction of the electron current flow, the fingers, when wrapped around the conductor, will point in the direction of the mag- netic lines of force. The illustration shows the application of the left-hand rule to determine the direction of the magnetic field about a conductor. RULE for A CONDUCTOR Remember that the left-hand rule is based on the electron theory of cur- rent flow (from negative to positive) and is used to determine the direction of the lines of force in an electromagnetic field. 1-52 MAGNETIC FIELDS Magnetic Field of a Loop or Coil Here is a point that you will find very important in the near future — a coil of wire carrying a current acts as a magnet. If a length of wire carrying a current is bent to form a loop, the lines of force around the conductor all leave at one side of the loop and enter at the other side. Thus the loop of wire carrying a current will act as a weak magnet having a north pole and a south pole. The north pole is on the side at which lines of force leave the loop and the south pole on the side at which they enter the loop. If you desire to make the magnetic field of the loop stronger, you can form the wire into a coil of many loops as shown. Now the individual fields of each loop are in series and form one strong magnetic field inside and out- side the loop. In the spaces between the turns, the lines of force are in opposition and cancel each other out. The coil acts as a strong bar magnet with the north pole being the end from which the lines of force leave. A left-hand rule also exists for coils to determine the direction of the magnetic field. If the fingers of the left hand are wrapped around the coil in the direction of the current flow, the thumb will point toward the north pole end of the coil. LEFT-HAND RULE FOR COILS 1-53 MAGNETIC FIELDS E lectromagnets Adding more turns to a current-carrying coil increases the number of lines of force, causing it to act as a stronger magnet. An increase in current also strengthens the magnetic field so that strong electromagnets have coils of many turns and carry as large a current as the wire size permits. In comparing coils using the same core or similar cores, a unit called the ampere-turn is used. This unit is the product of the current in amperes and the number of turns of wire. INCREASING CURRENT INCREASES FfELD STRENGTH Although the field strength of an electromagnet is increased both by using a large current flow and many turns to form the coil, these factors do not concentrate the field enough for use in a practical generator. To further increase the flux density, an iron core is inserted in the coil. Because the iron core offers much less reluctance (opposition) to lines of force than air, the flux density is greatly increased. 1-54 MAGNETIC FIELDS MAGNETIC FIELDS Demonstration — Magnetic Field Around a Conductor To demonstrate that a magnetic field exists around a current-carrying con- ductor, the instructor connects a heavy copper wire in series with a switch across a dry cell battery. The copper wire is bent to support itself verti- cally and then inserted through a hole in the lucite sheet, which is held in a horizontal position. With the switch closed, iron filings — which have the property of aligning themselves along the lines of force in a magnetic field — are sprinkled on the lucite. The lucite is tapped lightly to make it easier for the iron filings to fall into position. You see that the filings arrange themselves in concentric circles, showing that the magnetic lines of force form a circular pattern around the conduc- tor. To show that the circular pattern is actually the result of the mag- netic field, the instructor opens the switch and spreads the filings evenly over the cardboard, then repeats the demonstration. You see that, each time the circuit current flows, the filings arrange themselves to show the magnetic field. IRON FILINGS INDICATE CIRCULAR PATTERN OF MAGNETIC FIELD 1-56 MAGNETIC FIELDS Demonstration— Magnetic Field around a Conductor (continued) To demonstrate the direction of the magnetic field around the current- carrying conductor, a compass needle is used. A compass needle is nothing more than a small bar magnet which will line itself up with the lines of force in a magnetic field. You know from the pre- vious demonstration that the magnetic field is circular. Therefore, the compass needle always will be positioned at right angles to the current- carrying conductor. The iron filings are removed from the lucite, and the compass needle is placed on the lucite about 2 inches away from the conductor. With no cur- rent flowing, the north pole end of the compass needle will point to the earth's magnetic north pole. When current flows through the conductor, the compass needle lines itself up at right angles to a radius drawn from the conductor. As the compass needle is moved around the conductor, ob- serve that the needle always maintains itself at right angles to it. This proves that the magnetic field around the conductor is circular. Using the left-hand rule you can check the direction of the magnetic field which was indicated by the compass needle. The direction in which the fingers go around the conductor is the .same as that of the north pole of the compass needle. If the current through the conductor is reversed, the compass needle will point in the opposite direction, indicating that the direction of the magnetic field has reversed. Application of the left-hand rule will verify this observation. 1-57 MAGNETIC FIELDS Demonstration— Magnetic Fields Around a Coil To demonstrate the magnetic field of a coil of wire, a lucite board is used with No. 10 wire threaded through it to form a coil as shown. The rest of the circuit is the same as for the previous part of the demonstration. Iron filings are sprinkled on the lucite and current is passed through the coil. Tapping the lucite will cause the iron filings to line up parallel to the lines of force. Observe that the iron filings have formed the same pattern of a magnetic field that existed around a bar magnet. IRON FIL INGS DEMONSTRATING MAGN ETIC FIELD IHi PATTERN AROUND A COIL If the iron filings are removed, and the compass needle is placed inside the coil, the needle will line up along the axis of the coil with the north pole end of the compass pointing to the north pole end of the coil. Remem- ber that the lines of force inside a magnet or coil flow from the south pole to the north pole. The north pole end of the coil can be verified by using the left-hand rule for coils. If the compass is placed outside the coil and moved from the north pole to the south pole, the compass needle will fol- low the direction of a line of force as it moves from the north pole to the south pole. When the current through the coil is reversed, the compass needle will also reverse its direction. CHECKING DIRECTION OF MAGNETIC FIELD, USING COMPASS 1-58 MAGNETIC FIELDS Review of Electromagnetism ELECTROMAGNETIC FIELD — Cur- rent flowing through a wire generates a magnetic field whose direction is determined by the directionof the cur- rent flow. The direction of the gen- erated magnetic field is found by using the left-hand rule for a current- carrying conductor. MAGNETIC FIELD OF A LOOP OR COIL — A loop generates a magnetic field exactly the same as a bar mag- net. If many loops are added in series forming a coil, a stronger magnetic field is generated. The left-hand rule for a coil is used to determine the coil's magnetic polarity. FIELD STRENGTH — Increasing the number of turns of a coil increases the field strength and increasing the coil current also increases the field strength. An iron core may be in- serted to greatly concentrate the field (increase flux density) at the ends of the coil. The ampere -turn is the unit used in comparing the strength of magnetic fields. PERMANENT -MAGNET and ELEC - TROMAGNET FIELDS — Electro- magnet fields are much stronger than the permanent magnet type, and are used in most practical electrical ma- chinery. When electromagnets are used, the field strength can be varied by varying the amount of current flow through the field coils. 1-59 HOW CURRENT IS MEASURED How Electric Charges Are Measured In working with electric charges either standing still or in motion as cur- rent flow, you will need some unit for measuring the amount of electric charge. The basic unit of electrical charge is the electron but, since its charge is extremely small and the electron itself is so small that it cannot be seen, you will need to use a more practical unit of measurement. You are familiar with the measurement of grain, for example. Each kernel of grain is much too small to be used as a practical unit of measurement; therefore the bushel, containing several million kernels, is the practical unit used. Similarly, water is not measured by counting drops of water. Instead, a unit called the quart is used. For measuring electric charges the unit used is the coulomb, which is approximately 6.28 million, million, million electrons. The coulomb measures the quantity of electric charge or the number of electrons regardless of whether the charge is in motion or standing still. 1-60 HOW CURRENT IS MEASURED Units of Current Flow Current flow is a measure of how many electrons are passing through a material in a given length of time. The coulomb is a measure of the num- ber of electrons so that, by counting the coulombs which pass in a given amount of time, the current flow is measured. The unit of current flow is the ampere. One ampere of current is flowing when one coulomb of elec- trons passes through the material in one second, two amperes when two coulombs pass per second, etc. Since amperes mean coulombs per second, the ampere is a measure of rate at which electrons are moving through a material. The coulomb, which represents the number of electrons in a charge, is a measure of quantity. 1-61 HOW CURRENT IS MEASURED Measuring Units of Current Flow In working with electricity, a means of measuring current flow through a material is necessary. An ammeter does this: it indicates in amperes the number of electrons passing per second. When the amount of current flowing through a circuit is to be measured, the ammeter is always connected in series with the line that delivers cur- rent to the circuit; it will be damaged if it is connected in any other way. Because an ammeter indicates the rate of electron movement just as a meter in a water system shows the rate at which gallons of water are used, it follows that in order to show correctly the amount of current being used, the ammeter must be connected into the line (by breaking or opening the line to insert it). Without ammeter Ammeter connected in series with line to measure lamp current. Whenever you use an ammeter, the pointer indicates on the meter scale the number of amperes of current flowing, which is also the number of coulombs passing per second. THE AMMETER 1-62 HOW CURRENT IS MEASURED How Small Currents Are Measured While the ampere is the basic unit of measurement for current flow, it is not always a convenient unit to use. Current flows seldom exceed one thou- sand amperes but may often be as little as one one-thousandth of an am- pere. For measuring currents of less than one ampere some other unit is needed. A cup of water is not measured in gallons, nor is the flow of water from a fire hydrant measured in cups. In any kind of measurement a usable unit of measurement is needed. Since current flow seldom exceeds one thousand amperes, the ampere can be used satisfactorily as the unit for currents in excess of one ampere. However, it is not convenient as the unit for currents of less than one ampere. If the current flow is between one -thousandth of an ampere and one am- pere, the unit of measure used is the milliampere (abbreviated ma.), which is equal to one -thousandth ampere. For current flow of less than one- thousandth ampere, the unit used is the microampere, which is equal to one- millionth ampere. Meters used for measuring milliamperes of current are called milliammeters, while meters used for measuring microamperes of current are called microammeters. Units of measurement are subdivided in such a way that a quantity expressed in one unit may be readily changed to another unit, either larger or smaller. For example, in volume measure one-half gallon equals two quarts, and four pints also equals two quarts. The relation between the different units of current is indicated below. HOW CURRENT IS MEASURED How Units of Current Are Changed In order to work with electricity, you must be able to change from one unit of current to another. Since a milliampere (ma.) is one -thousandth of an ampere, milliamperes can be changed to amperes by moving the decimal point three places to the left. For example, 35 milliamperes is equal to 0.035 ampere . There are two steps required in order to arrive at the correct answer. First, the original position of the decimal point must be located. The decimal is then moved three places to the left, changing the unit from milliamperes to amperes. If no decimal point is given with the number, it is always understood to follow the last number in the quantity. In the example given, the reference decimal point is after the number 5, and to change from milliamperes to amperes it must be moved three places to the left. Since there are only two whole numbers to the left of the deci- mal point, a zero must be added to the left of the number to provide for a third place as shown. When changing amperes to milliamperes you move the decimal point to the right instead of the left. For example, 0.125 ampere equals 125 milliam- peres and 16 amperes equals 16,000 milliamperes. In these examples, the decimal point is moved three places to the right of its reference position, with three zeros added in the second example to provide the necessary decimal places. CHANGING MILLIAMPERES TO AMPERES 35 * ? s4*ttfine Move decimal point three places to the left. MILLIAMPERES = .035 AMPERE CHANGING AMPERES TO MILLIAMPERES J 25 rftKfrent - ? TKUlicimptntA Move decimal point three places to the right. .125aMPERE = 125. MILLIAMPERES 1-64 HOW CURRENT IS MEASURED How Units of Current Are Changed (continued) Suppose that you are working with a current of 125 microamperes and you need to express this current in amperes. If you are changing from a large unit to a small unit, the decimal point is moved to the right, while to change from a small unit to a large unit, the decimal point is moved to the left. Since a microampere is one-millionth ampere, the ampere is the larger unit. Then changing microamperes to amperes is a change from small to large units and the decimal point should be moved to the left. In order to change millionths to units, the decimal point must be moved six decimal places to the left so that 125 microamperes equals 0.000125 ampere . The reference point in 125 microamperes is after the 5, and in order to move the decimal point six places to the left you must add three zeros ahead of the number 125. When changing microamperes to milliamperes, the deci- mal point is moved only three places to the left, and thus 125 microam- peres equals 0.125 milliampere . If your original current is in amperes and you want to express it in micro- amperes, the decimal point should be moved six places to the right. For example, 3 amperes equals 3,000,000 microamperes,because the reference decimal point after the 3 is moved six places to the right with the six zeros added to provide the necessary places. To change milliamperes to micro- amperes the decimal point should be moved three places to the right. For example, 125 milliamperes equals 125,000 microamperes, with the three zeros added to provide the necessary decimal places. 125 . Microamperes = .125 Milliampere AMPERES TO MICROAMPERES MILLIAMPERES TO MICROAMPERES Move Decimal Point Six Places to the Right. 3 . Amperes “ 3 , 000 , 000 > Microamperes Move Decimal Point Three Places to the Right. 12 5 . Milliamperes = 125 , 000 . Microamperes 1-65 HOW CURRENT IS MEASURED Milliammeters and Microammeters An ammeter having a meter scale range of 0-1 ampere is actually a milli- ammeter with a range of 0-1000 milliamperes. Fractions are seldom used in electricity so that, on the 0-1 ampere range, a meter reading of 1/2 am- pere is given as 0.5 ampere or 500 milliamperes. For ranges less than 1 ampere, milliammeters and microammeters are used to measure current. If you are using currents between 1 milliampere and 1000 milliamperes, milliammeters are used to measure the amount of current. For currents of less than 1 milliampere, microammeters of the correct range are used, Very small currents of 1 microampere or less are measured on special laboratory type instruments called galvanometers. You will not normally use the galvanometer, since the currents used in electrical equipment are between 100 microamperes and 100 amperes and thus can be measured with a microammeter, milliammeter or ammeter of the correct range. Meter scale ranges for milliammeters and microammeters, like ammeters, are in multiples of 5 or 10 since these multiples are easily changed to oth- er units. In using a meter to measure current, the maximum reading of the meter range should always be higher than the maximum current to be measured. A safe method of current measurement is to start with a meter having a range much greater than you expect to measure, in order to determine the correct meter to use. 1-66 HOW CURRENT IS MEASURED How Meter Scales Are Read When you work with electricity it is necessary that you take accurate me- ter readings, to determine whether equipment is working properly, and to discover what is wrong with equipment which is not operating correctly. Many factors can cause meter readings to be inaccurate and it is necessary to keep them in mind whenever you use a meter. You will find the usable range of a meter scale does not include the extreme ends of the scale. For nearly all meters, the most accurate readings' are those taken near the center of the scale. When current is measured with an ammeter, milliam- meter or microammeter, the range of the meter used should be chosen to give a reading as near to mid-scale as possible. All meters cannot be used in both horizontal and vertical positions. Due to the mechanical construction of many meters, the accuracy will vary consid- erably with the position of the meter. Normally, panel-mounting type me- ters are calibrated and adjusted for use in a vertical position. Meters used in many test sets and in some electrical equipment are made for use in a horizontal position. A zero set adjustment on the front of the meter is used to set the meter needle at zero on the scale when no current is flowing. This adjustment is made with a small screwdriver and should be checked when using a meter, particularly if the vertical or horizontal position of the meter is changed. 1-67 HOW CURRENT IS MEASURED How Meter Scales Are Read (continued) Meter scales used to measure current are divided Into equal divisions, usually with a total of between thirty and fifty divisions. The meter should always be read from a position at right angles to the meter face. Since the meter divisions are small and the meter pointer is raised above the scale, reading the pointer position from an angle will result in an inaccurate reading, often as much as an entire scale division. This type of incorrect reading is called "parallax." Most meters are slightly inaccurate due to the meter construction, and additional error from a parallax reading may result in a very inaccurate reading. When the meter pointer reads a value of current between two divisions of the scale, usually the nearest division is used as the meter reading. How- ever, if a more accurate reading is desired, the position of the pointer be- tween the divisions is estimated, and the deflection between the scale di- visions is added to the lower scale division. Estimating the pointer posi- tion is called "Interpolation," and you will use this process in many other ways in working with electricity. 1-68 HOW CURRENT IS MEASURED Usable Meter Range The range of an ammeter indicates the maximum current which can be measured with the meter. Current in excess of this value will cause ser- ious damage to the meter. If an ammeter has a range of 0-15 amp, it will measure any current flow which does not exceed 15 amperes, but a current greater than 15 amperes will damage the meter. While the meter scale may have a range of 0-15 amperes, its useful range for purposes of measurement will be from about 1 ampere to 14 amperes. When this meter scale indicates a current of 15 amperes, the actual current may be much greater but the meter can only indicate to its maximum range. For that reason the useful maximum range of any meter is slightly less than the maximum range of the meter scale. A current of 0.1 ampere on this meter scale would be very difficult to read since it would not cause the meter needle to move far enough from zero to obtain a definite reading. Smaller currents such as 0:001 ampere would not cause the meter needle to move and thus could not be measured at all with this meter. The useful minimum range of a meter never extends down to zero, but extends instead only to the point at which the reading can be distinguished from zero. Ammeter ranges are usually in multiples of 5 or 10 such as 0-5 amperes, 0-50 amperes, etc. Ranges above 0-100 amperes are not common since currents in excess of 100 amperes are seldom used. 1-69 HOW CURRENT IS MEASURED Demonstration — Ammeter Ranges To show the importance of selecting the proper meter range for current measurement, the instructor first connects two dry cells in series to form a battery. Then the positive terminal of the 0-10 amp range ammeter is connected with a length of pushback wire to the positive terminal of the battery. Next, a lamp socket is connected between the negative terminal of the ammeter and the negative terminal of the battery. The lamp bulb is used as a switch to control current flow. The lamp lights when it is inserted in the socket, and the meter pointer moves slightly — indicating a current flow. The meter reading indicates that the current flow is very low for the meter range used. The pointer is near the low end of the meter scale and the current cannot be read accurately. Next, the 0-1 amp range is used instead of the 0-10 amp range. Observe that when the lamp is inserted, its light indicates the current flow is the same as before. However, the meter reading now is near the midscale po- sition of the meter scale, indicating a current flow of slightly more than one -half ampere. Since the reading is near midscale, this is the correct meter range for measuring the current. 1-70 HOW CURRENT IS MEASURED Demonstration — Ammeter Ranges (continued) To show the effect of using a meter having too low a range, the instructor next uses the 0-500 ma. range instead of the 0-1 amp range. Because the current flow is greater than the maximum range of the meter, the pointer deflects beyond the range of the scale and you are unable to read the amount of current flow. If this excess current flows through the meter for any length of time, it will cause serious damage to the meter. For that reason it is more important that the meter range be high enough, than that it be low enough, to obtain a good reading. r FINDING THE 1 ^ CORRECT METER RANGE A To find the correct meter range where the current expected is not known, you should always start with a meter having a high maximum range and re- place it with meters having lower ranges until a meter reading is obtained near mid-scale. 1-71 HOW CURRENT IS MEASURED Demonstration — Reading Meter Scales You have observed that the correct ammeter range to use in measuring the current flow through the lamp is the 0-1 amp range. To show the effects of parallax on meter readings, the instructor uses a 0-1 amp range instead of the 0-500 ma. range. Now, with the lamp inserted, the meter indicates a current flow Somewhat greater than one-half ampere. The instructor will ask several trainees to read the meter simultaneously from different posi- tions and record their readings. Notice that the readings taken at wide an- gles differ considerably from those obtained directly in front of the meter. Now the entire class reads the meter and interpolates the reading by esti- mating between scale divisions. Since the meter scale permits reading to two places directly, the interpolation is the third figure of the reading. For example, the scale divisions between .6 and .7 are .62, .64, .66 and .68. If the meter pointer is between the .62 and .64 scale divisions and is halfway between these divisions, the meter reading is .630 ampere. A reading one-fourth of the way past the .62 division is .625 ampere, three- fourths of the way past the .62 division is .635 ampere, etc. Observe that the estimated or interpolated value obtained by everyone is more ac- curate than the nearest scale division, but there is disagreement among the interpolated readings. 1-72 HOW CURRENT IS MEASURED Review of How Current Is Measured To review what you have found out about how current is measured, consider some of the important facts you have studied and seen demonstrated. 1. AMPERE — Unit of rate of flow of elec- trons, equal to 1 coulomb per second. 2. MILT -TAMPERE — A unit of current equal to one -thousandth ampere. 3. MICROAMPERE — A unit of current equal to one -millionth ampere. 4, AMMETER — A meter used to measure currents of one ampere and greater. 5. MILLIAMME TER — A meter used to measure currents between one -thousandth ampere and one ampere. 6. MICROAMMETER — A meter used to measure currents between one -millionth ampere and one -thousandth ampere. 7. PARALLAX — Meter reading error due to taking a reading from an angle. 8. INTERPOLATION — Estimatingthe me- ter reading between two scale divisions. METER READING IS Interpolation 1-73 HOW A METER WORKS The Basic Meter Movement AMMETERS You have used meters for sometime now to show you whether or not an electric current was flowing and how much current was flowing. As you pro- ceed further with your work in electricity, you will find yourself using me- ters more and more often. Meters are the right hand of anyone working in electricity or electronics, so now is the time for you to find out how they operate. All the meters you have used and nearly all the meters youwill ever use are made with the same type of meter "works" or movement. This meter movement is based on the principles of an electric current- measuring device called the "moving coil galvanometer." Nearly all modern meters use the moving coil galvanometer as a basic meter movement, so once you know how it works you will have no trouble understanding all the meters you will be using in the future. 1-74 HOW A METER WORKS The Basic Meter Movement (continued) The galvanometer works on the principle of magnetic attraction and re- pulsion. According to this principle, which you have already learned, like poles repel each other and unlike poles attract each other. This means that two magnetic North poles will repel each other as will two magnetic South poles, while a North pole and South pole will attract one another. You can see this very well if you suspend a bar magnet on a rigidly mounted shaft, between the poles of a horseshoe magnet. If the bar magnet is allowed to turn freely, you will find that it turns until its North pole is as close as possible to the South pole of the horseshoe magnet and its South pole is as close as possible to the North pole of the horseshoe magnet. If you turn the bar magnet to a different position, you will feel it trying to turn back to the position where the opposite poles are as near as possible to each other. The further you try to turn the bar magnet away from this position, the greater force you will feel. The greatest force will be felt when you turn the bar magnet to the position in which the like poles of each magnet are as close as possible to each other. HOW A METER WORKS The Basic Meter Movement (continued) The forces of attraction and repulsion between magnetic poles become greater when stronger magnets are used. You can see this if you attach a spring to the bar magnet in such a way that the spring will have no ten- sion when the North poles of the two magnets are as close as possible to each other. With the magnets in this position the bar magnet would nor- mally turn freely to a position which would bring its North pole as close as possible to the South pole of the horseshoe magnet. With the spring at- tached it will turn only part way, to a position where its turning force is balanced by the force of the spring. If you were to replace the bar magnet with a stronger magnet, the force of repulsion between the like poles would be greater and the bar magnet would turn further against the force of the spring. HOW A METER WORKS The Basic Meter Movement (continued) If you remove the bar magnet and replace it with a coil of wire, you have a galvanometer. Whenever an electric current flowsthrough this coilof wire, it acts like a magnet. The strength of this wire coil magnet depends on the size, shape and number of turns in the coil and the amount of electric cur- rent flowing through the coil. If the coil itself is not changed in any way, the magnetic strength of the coil will depend on the amount of current flow- ing through the coil. The greater the current flow in the coil, the stronger the magnetic strength of the wire coil magnet. If there is no current flow in the coil, it will have no magnetic strength and the coil will turn to a position where there will be no tension on the spring. If you cause a small electric current to flow through the coil, the coil be- comes a magnet and the magnetic forces— between the wire coil magnet and the horseshoe magnet — cause the coil to turn until the magnetic turning force is balanced by the force due to tension in the spring. When a larger current is made to flow through the coil, the magnet strength of the coil is increased and the wire coil turns further against the spring tension. HOW A METER WORKS The Basic Meter Movement (continued) When you want to find out how much current is flowing in a circuit, all you need to do is to connect the coil into the circuit and measure the angle through which the coil turns away from its position at rest. To measure this angle, and to calculate the amount of electric current which causes the coil to turn through this angle, is very difficult. However, by con- necting a pointer to the coil and adding a scale for the pointer to travel across, you can read the amount of current directly from the scale. Now that you have added a scale and a pointer, you have a basic DC meter, known as the D' Arsonval-type movement, which depends upon the operation of magnets and their magnetic fields. Actually, there are two magnets in this type of meter; one a stationary permanent horseshoe magnet, the other an electromagnet. The electromagnet consists of turns of wire wound on a frame, and the frame is mounted on a shaft fitted between two permanently- mounted jewel bearings. A lightweight pointer is attached to the coil, and turns with it to indicate the amount of current flow. Current passing through the coil causes it to act as a magnet with poles being attracted and repelled by those of the horseshoe magnet. The strength of the magnetic field about the coil depends upon the amount of current flow. A greater Current produces a stronger field, resulting in greater forces of attraction and repulsion between the coil sides and the magnet's poles. The magnetic forces of attraction and repulsion cause the coil to turn so that the unlike poles of the coil and magnet will be brought together. As the coil current increases, the coil becomes a stronger magnet and turns further because of the greater magnetic forces between the coil and mag- net poles. Since the amount by which the coil turns depends upon the amount of coil current, the meter indicates the current flow directly. 1-78 HOW A METER WORKS Meter Movement Considerations While galvanometers are useful in laboratory measurements of extremely small currents, they are not portable, compact or rugged enough for use in military equipment. A modern meter movement uses the principles of the galvanometer but is portable, compact, rugged and easy to read. The coil is mounted on a shaft fitted between two permanently-mounted jewel bearings. To indicate the amount of current flow a lightweight pointer is attached to the coil and turns with the coil. Balance springs on each end of the shaft exert opposite turning forces on the coiiand, by adjusting the tension of one spring, the meter pointer may be adjusted to read zero on the meter scale. Since temperature change affects both coil springs equally, the turning effect of the springs on the meter coil is canceled out. As the meter coil turns, one spring tightens to provide a retarding force, while the other spring releases. its tension. In addition to providing tension, the springs are used to carry current from the meter terminals through the moving coil. In order that the turning force will increase uniformly as the current in- creases, the horseshoe magnet poles are shaped to form semi-circles. This brings the coil as near as possible to the north and south poles of the permanent magnet. The amount of current required to turn the meter pointer to full-scale deflection depends upon the magnet strength and the 1-79 HOW A METER WORKS How Meter Ranges Are Changed Meter ranges could be changed by using magnets of different strength or by changing the number of turns in the coil, since either of these changes would alter the amount of current needed for full-scale deflection. How- ever, the wire used in the coil must always be large enough to carry the maximum current of the range the meter is intended for, and therefore changing the wire size would only be practical in the small current ranges, since large wire cannot be used as a moving coil. To keep the wire size and the coil small, basic meter movements are normally limited to a range of 1 milliampere or less. Also, for using a meter for more than one range, it is impractical to change the magnet or the coil each time the range is changed. For measuring large currents a low range meter is used with a shunt, which is a heavy wire connected across the meter terminals to carry mpst of the current. This shunt allows only a small part of the current to actually flow through the meter coil. Usually a 0-1 milliampere meter is used, with the proper-sized shunt connected across its terminals to achieve the desired range. The 0-1 milliammeter is a basic meter movement which you will find in various types of meters you will use. 1-80 HOW A METER WORKS Multi-Range Ammeters You have seen that you can change the range of an ammeter by the use of shunts. The range will vary according to the resistive value of the shunt. Some ammeters are built with a number of internal shunts, and a switching arrangement which is used to parallel different shunts across the meter movement to measure different currents. Thus a single meter movement can be used as a multi-range ammeter. A scale for each range is painted on the meter face. The diagram below shows a multi-range ammeter with a 0-3, 0-30, 0-300 ampere range. Note the three scales on the meter face. When a multi-range ammeter is used to measure an unknown current, the highest range is always used first, then the next highest range, and so on until the needle is positioned about midscale. In this way you can be as- sured that the current is not excessive for the meter range, and you will never have the unfortunate experience of burning out a meter movement, or of wrapping the needle around the stop-peg. Some multimeters use external shunts, and do away with internal shunts and the switching arrangement. Changing range for such a meter involves shunt- ing it with the appropriate shunt. In the diagram the ammeter is calibrated to read 30 amperes full-scale by shunt- ing it with the 30-ampere shunt. \AMFERE3 300-ampere shunt 30 -ampere shunt 1-81 HOW A METER WORKS Review of Meter Movement It is very possible that you may never need to repair a meter but, in order to properly use and care for meters, you need to know how a meter works. Suppose you review what you have studied. METER COIL — Moving coil which acts as a magnet when current flows in the coil. METER MOVEMENT — Current- measuring instrument consisting of a moving coil suspended between the poles of a horseshoe magnet. Cur- rent in the coil causes the coil to turn. BASIC AMMETER MOVEMENT — 0-1 ma. milliammeter movement with shunt wire across the meter terminals to in- crease the meter scale range. MULTI-RANGE AMMETER — A single meter movement used for measuring different current ranges. Each range re- quires a different shunt. The shunts may be inside the meter movement and con- trolled by a switching arrangement or they may be external, in which case they are connected in parallel with the meter bind- ing posts. 1-82 WHAT CAUSES CURRENT FLOW— EMF What EMF Is Current flow takes place whenever most of the electron movement in a material is in one direction. You have found out that this movement is from a (-) charge to a (+) charge and occurs only as long as a difference in charge exists. To create a charge, electrons must be moved, either to cause an excess or alack of electrons at the point where the charge is to exist. A charge may be created by any of the six sources of electricity which you have studied about previously. These sources furnish the energy required to do the work of moving electrons to form a charge. Regardless of the kind of energy used to create a charge, it is changed to electrical energy once the charge is created; and the amount of electrical energy existing in the charge is exactly equal to the amount of the source energy required to create this charge. When the current flows, the electrical energy of the charges is utilized to move electrons from less positive to more positive charges. This elec- trical energy is called electromotive force (emf) and is the moving force which causes current flow. Electrons may be moved to cause a charge by using energy from any of the six sources of electricity; but, when electrons move from one charge to another as current flow, the moving force is emf. 1-83 WHAT CAUSES CURRENT FLOW— EMF What EMF Is (continued) An electric charge, whether positive or negative, represents a reserve of energy. This reserve energy is potential energy as long as it is not being used. The potential energy of a charge is equal to the amount of work done to create the charge, and the unit used to measure this work is the volt . The electromotive force of a charge is equal to the potential of the charge and is expressed in volts. When two unequal charges exist, the electromotive force between the charges is equal to the difference in potential of the two charges. Since the potential of each charge is expressed in volts the difference in poten- tial is also expressed in volts. The difference in potential between two charges is the electromotive force acting between the charges— commonly called voltage. Voltage or a difference in potential exists between any two charges which are not exactly equal. Even an uncharged body has a potential difference with respect to a charged body; it is positive with respect to a negative charge and negative with respect to a positive charge. Voltage exists, for example, between two unequal positive charges or between two unequal negative charges. Thus voltage is purely relative and is not used to ex- press the actual amount of charge, but rather to compare one charge to another and indicate the electromotive force between the two charges being compared. VOtTAGE IS THt ^HREHCE ih poTEHt'N- , 100 I VOLTS I r 9 *100 VOLT< L< !> J 1-84 WHAT CAUSES CURRENT FLOW— EMF How EMF Is Maintained Of the six sources of electricity, you will usually use only magnetism and chemical action. Electric charges obtained from friction, pressure, heat and light are only used in special applications and are never used as a source of electric power. In order to cause continuous current flow, electric charges must be main- tained so that the difference of potential remains the same at all times. At the terminals of a battery, opposite charges exist caused by the chemi- cal action within the battery, and as current flows from the (-) terminal to the (+) terminal the chemical action maintains the charges at their original value. A generator acts in the same manner, with the action of a wire moving through a magnetic field maintaining a constant charge on each of the generator terminals. The voltage between the generator or battery terminals remains constantand the charges on the terminals never become equal to each other as long as the chemical action continues in the battery and as long as the generator wire continues to move through the magnetic field. Battery J Battery discharging | EMF maintained If the charges were not maintained at the terminals, as in the case of two charged bars shown below, current flow from the (-) terminal to the (+) terminal would cause the two charges to become equal as the excess elec- trons of the (-) charge moved to the (+) charge. The voltage between the terminals then would fall to zero volts and current flow would no longer take place. Wire Charged bars Bars discharging EMF not maintained 1-85 WHAT CAUSES CURRENT FLOW— EMF Voltage and Current Flow Whenever two points of unequal charge are connected, a current flows from the more negative to the more positive charge. The greater the emf or voltage between the charges, the greater the amount of current flow. Electrical equipment is designed to operate with a certain amount of cur- rent flow, and when this amount is exceeded the equipment may be dam- aged. You have seen all kinds of equipment such as electric lamps, motors, radios, etc. with the voltage rating indicated. The voltage will differ on certain types of equipment, but it is usually 110 volts. This rating on a lamp, for example, means that 110 volts will cause the cor- rect current flow. Using a higher voltage will result in a greater current flow and "burn out" the lamp, while a lower voltage will not cause enough current flow. If a motor is designed to operate on 110 volts and you connect it to a 220- volt electric power line, the motor will be "burned out" due to excessive current flow; but the same motor placed across a 50-volt line will not op- erate properly because not enough current will flow. While current flow makes equipment work, it takes emf or voltage to cause the current to 1-86 WHAT CAUSES CURRENT FLOW— EMF Voltage and Current Flow (continued) Electromotive force— voltage — is used like any other type of force. To drive a nail you might use any number of different size hammers but only one size would furnish exactly the right amount of force for a par- ticular nail. You would not use a sledge hammer to drive a tack nor a tack hammer to drive a large spike. Choosing the correct size hammer to drive a nail is just as important as finding the correct size nail to use for a given job. Similarly, electrical devices and equipment operate best when the correct current flows, but for a given device or equipment you must choose the correct amount of voltage to cause just the right amount of current flow. Too large a voltage will cause too much current flow, while too small a voltage will not cause enough current flow. 1-87 HOW VOLTAGE IS MEASURED Units of Voltage The electromotive force between two unequal charges is usually expressed in volts but, when the difference in potential is only a fraction of a volt or is more than a thousand volts, other units are used. For voltages of Jess than one volt, millivolts and microvolts are used, just as milliamperes and microamperes are used to express currents less than one ampere. While current seldom exceeds one thousand amperes, voltage often exceeds one thousand volts, so that the kilovolt— equal to one thousand volts— is used as the unit of measurement. When the potential difference between two charges is between one -thousandth of a volt arid one volt, the unit of measure is the millivolt; when it is between one-millionth volt and one- thousandth volt, the unit is the microvolt. Meters for measuring voltage have scale ranges in microvolts, millivolts, volts and kilovolts, depending on the units of voltage to be measured. Or- dinarily you will work with voltages between 1 and 500 volts and use the volt as a unit. Voltages of less than 1 volt and more than 500 volts are not used except in special applications of electrical equipment. *VH^ex IN RESISTANCE 1-102 WHAT CONTROLS CURRENT FLOW— RESISTANCE Factors Controlling Resistance— Length The next factor greatly affecting the resistance of a conductor is its length. The longer the length, the greater the resistance; and the shorter the length, the lower the resistance. You know that a material such as iron resists the flow of electric current, simply because of the manner in which each atom holds on to its outer electrons. It is easy to see that the more iron you put in the path of an electric current, the less will be the current flow. Suppose you were to connect an iron wire four inches long and 1/100 inch thick in series with an ammeter. As soon as you connect this across a dry cell, a certain amount of current will flow. The amount of current that flows depends upon the voltage of the dry cell and the number of times the electron gets "stuck" or "attracted" by the atoms in its path between the terminals of the voltage source. If you were to double the length of the iron wire, making it eight inches long, there would be twice as much iron in the path of the electric current and the electron would be held by twice as many atoms in its path between the terminals of the voltage source. By doubling the length of the electric current path between the terminals of the dry cell, you have put twice as many attractions in the way, and you have doubled the resistance. to electric current flow. The shorter the length of a given type of con- ductor, the less resistance it will offer to the flow of an electric current. 1-103 WHAT CONTROLS CURRENT FLOW— RESISTANCE Factors Controlling Resistance— -Cross-Sectional Area Another factor affecting the resistance of a conductor is its cross-sectional area. To understand what cross-sectional area means, suppose you imag- ine a wire cleanly cut across any part of its length. The area of the cut face of the wire is the cross-sectional area. The greater this area, the lower is the resistance of the wire; and the smaller this area, the higher is the resistance of the wire. To see how this works suppose that you were to connect an iron wire four inches long and 1/100 inch thick in series with an ammeter. As soon as you connect this across a dry cell, a certain amount of current will flow. The amount of current that flows depends upon the voltage of the dry cell and the path of iron wire put in the way of the current flow between the ter- minals of the voltage source. You can see that the electric current has a pretty narrow wire (1/100 inch thick) to travel through. If you were to re- move the iron wire and replace it with another wire which has the same length but twice the cross-sectional area, the current flow would double. This happens because you now have a "wider path" for the electric current to flow through— twice as many free electrons are available to make up the current which has the same length of path to flow through. The larger the cross-sectional area of a conductor, the lower the resist- ance; and the smaller the cross-sectional area, the higher the resistance. WHAT CONTROLS CURRENT FLOW— RESISTANCE Factors Controlling Resistance — Temperature The final factor affecting the resistance of a conductor is its temperature. For most materials the hotter the material, the more resistance it offers to the flow of an electric current; and the colder the material, the less re- sistance it offers to the flow of an electric current. This effect comes about because a change in the temperature of a material changes the ease with which that material releases its outer electrons. You can see this effect by connecting a length of resistance wire, a switch and a dry cell in series. When you close the switch a certain amount of electric current will flow through the wire. In a short time the wire will begin to heat up. As the wire begins to heat, its atoms hold more tightly onto the outer electrons and the resistance goes up. You can see the re- sistance go up by watching the meter; as the wire gets hotter and hotter, the resistance to the electric current rises and the meter reading will fall lower and lower. When the wire has reached its maximum heat, its resistance will stop increasing and the meter reading will remain at a steady value. Some materials such as carbon and electrolytic solutions lower their re- sistance to an electric current as the temperature increases, and the elec- tric current increases as the temperature increases. The effect of tem- perature upon resistance varies with the type of material — in materials such as copper and aluminum, it is very slight. The effect of temperature on resistance is the least important of the four factors controlling re- sistance — material, length, cross-sectional area and temperature. 1-105 WHAT CONTROLS CURRENT FLOW— RESISTANCE Units of Resistance To measure current the ampere is used as a unit of measure and to measure voltage the volt is used. These units are necessary in order to compare different currents and different voltages. In the same manner, a unit of measure is needed to compare the resistance of different conduc- tors. The basic unit of resistance is the ohm, equal to that resistance which will allow exactly one ampere of current to flow when one volt of emf is applied across the resistance. Suppose you connect a copper wire across a voltage source of 1 volt and adjust the length of the wire until the current flow through the wire is exactly one ampere. The resistance of the length of copper wire then is exactly 1 ohm. If you were to use wire of any other materials — iron, silver, etc. — you would find that the wire length and size would not be the same as that for copper. However, in each case you could find a length of the wire which would allow exactly 1 ampere of current to flow when connected across a 1-volt voltage source, and each of these lengths would have a resistance of 1 ohm. The resistances of other lengths and sizes of wire are compared to these 1-ohm lengths, and their resistances are expressed in ohms. Like other parts of a circuit, a symbol is used to indicate resistance. — VW\AM — RESISTANCE 1-106 WHAT CONTROLS CURRENT FLOW— RESISTANCE Units of Resistance (continued) Most of the time you will use resistance values which can be expressed in ohms but for certain special applications you may use small values of less than one ohm or values greater than one million ohms. Fractional values of resistance are expressed in microhms and very large values are expressed in megohms. One microhm equals one-millionth ohm, while one megohm is equal to a million ohms. Units of resistance are changed in the same manner as units of current or voltage. To change microhms to ohms the decimal point is moved six places to the left, and to change ohms to microhms the decimal point is moved six places to the right. To change megohms to ohms the decimal point is moved six places to the right, and to change ohms to megohms it is moved six places to the left. For resistances between one thousand and one million ohms the unit used is the kilohm (K) which is always abbreviated in use. Ten kilohms is written 10K and equals 10,000 ohms. To change kilohms to ohms the decimal point is moved three places to the right, and to change ohms to kilohms the decimal point is moved three places to the left. MICROHMS TO OHMS OHMS TO MICROHMS Move the decimal point 6 places to Move the decimal point 6 places to the left the right 35000 microhms = .035 ohm 3.6 ohms = 3,600,000 microhms KILOHMS TO OHMS OH MS TO KILOHMS Move the decimal point 3 places to Move the decimal point 3 places to the right the left 6 kilohms = 6000 ohms 6530 ohms = 6.530 kilohms MEGOHMS TO OHMS OHMS TO MEGOHMS Move the decimal point 6 places to Move the decimal point 6 places to the right the left 2.7 megohms = 2,700,000 ohms 650,000 ohms = .65 megohm 1-107 WHAT CONTROLS CURRENT FLOW— RESISTANCE How Resistance Is Measured Voltmeters and ammeters are meters you are familiar with and have used to measure voltage and current. Meters used to measure resistance are called ohmmeters. These meters differ from ammeters and volt- meters particularly in that the scale divisions are not equally spaced, and the meter requires a built-in battery for proper operation. When using the ohmmeter, no voltage should be present across the resistance being measured except that of the ohmmeter battery; otherwise, the ohmmeter will be damaged. Ohmmeter ranges usually vary from 0-1000 ohms to 0-10 megohms. The accuracy of the meter readings decreases at the maximum end of each scale, particularly for the megohm ranges, because the scale divisions become so closely spaced that an accurate reading cannot be obtained. Unlike other meters, the zero end of the ohmmeter scale is at full-scale deflection of the meter pointer. I O&mtnet&i *Dial Scale • Special ohmmeters called "meggers" are required to measure values of resistance over 10 megohms, since the built-in voltage required is very high for ranges above 10 megohms. Some meggers use high voltage batteries and others use a special type of hand generator to obtain the necessary voltage. While ohmmeters are used to measure the resistance of conductors, the most important use of meggers is to measure and test insulation resistance. 1-108 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistors— Construction and Properties There is a certain amount of resistance in all of the electrical equipment which you use. However, sometimes this resistance is not enough to con- trol the flow of current to the extent desired. When additional control is required — for example, when starting a motor — resistance is purposely added to that of the equipment. Devices which are used to introduce addi- tional resistance are called resistors. You will use a wide variety of resistors, some of which have a fixed value and others which are variable. All resistors are made either of special resistance wire, of graphite (carbon) composition, or of metal film. Wire- wound resistors are usually used to control large currents while carbon resistors control currents which are relatively small. Vitreous enameled wire-wound resistors are constructed by winding re- sistance wire on a porcelain base, attaching the wire ends to metal termi- nals, and coating the wire and base with powdered glass and baked enamel to protect the wire and conduct heat away from it. Fixed wire-wound resistors are also used which have coating other than vitreous enamel. Precision wound resistors of Manganin wire are used where the resistance value must be very accurate such as in test instruments. Precision Wire-wound Resistors 1-109 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistors— Construction and Properties (continued) Carbon resistors are constructed of a rod of compressed graphite and binding material, with wire leads attached to each end of the rod. The rod is then either painted or covered by an insulating coating of ceramic. Leads used for this type of resistor are called pigtail leads. CARBON RESISTORS Some carbon resistors are made by coating a porcelain tube with a carbon film, and in some cases the film is coated in a spiral similar to winding a wire around the tube. The carbon coating is covered with baked enamel, for protection and to conduct heat away from the carbon film so that it does not overheat and burn out. LARGE CARBON RESISTORS Metal film resistors are constructed in the same manner as spiral-coated carbon resistors except that the film is metallic instead of carbon. 1-110 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistors— Construction and Properties (continued) You will not always use resistors of fixed value, since very often you will need to change resistance while the equipment is in operation. To do this you will use both carbon and wire-wound variable resistors, depending on the amount of current to be controlled— wire-wound for large currents and carbon for small currents. Wire-wound variable resistors are constructed by winding resistance wire on a porcelain or bakelite circular form, with a contact arm which can be adjusted to any position on the circular form by means of a rotating shaft. A lead connected to this movable contact can then be used, with one or both of the end leads, to vary the resistance used. Variable Contact Terminal End Terminals ■Cover Variable Slider Contact Resistance Element ^Rotating ^ Shaft WIRE-WOUND VARIABLE RESISTORS For controlling small currents, carbon variable resistors are constructed by depositing a carbon compound on a fiber disk. A contact on a movable arm acts to vary the resistance as the arm shaft is turned. End Terminals Variable Contact Terminal Cover Variable Slider Contact Resistance Element Rotating Shaft CARBON VARIABLE RESISTORS 1-111 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistors- Construction and Properties (continued) Variable resistors of either type— wire-wound or carbon— may usually be used in two ways, as a rheostat or as a potentiometer. Some variable re- sistors have only two terminals and these can only be used as rheostats. A three- terminal variable resistor connected as a rheostat has only two leads connected to the electrical circuit and is used to vary the resistance between these two leads. If the variable contact terminal and one end terminal are connected together directly and act as only one lead in the circuit, the variable resistor acts as a rheostat. TWO-TERMINAL THREE-TERMINAL VARIABLE RESISTORS VARIABLE RESISTORS lllt&as ■: 1 ^ If the three terminals of a variable resistor each connect to different parts of the circuit, it is connected as a potentiometer. With this kind of connec- tion the resistance between the end terminals is always the same, and the variable arm provides a contact which can be moved to any position be- tween the end terminals. A potentiometer does notvary the total resistance between the end terminals but, instead, varies the amount of resistance between each end and the center contact with both resistances changing as the variable contact is moved— one increasing as the other decreases. III- ww/w -W/W/V-J Potentiometer Load 1-112 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistor Color Code You can find the resistance value of any resistor by using an ohmmeter, but in some cases it is easier to find the value of a resistor by its mark- ing. Most wire-wound resistors have the resistance value printed in ohms on the body of the resistor. If they are not marked in this manner, you must use an ohmmeter. Many carbon resistors also have their values printed on them, but carbon resistors are often mounted so that you can- notread the printed marking. Also, heat often discolors the resistor body, making it impossible to read a printed marking, and in addition, some carbon resistors are so small that a printed marking could not be read. To make the value of carbon resistors easy to read, a color code mark- ing is used. Carbon resistors are of two types, radial and axial, which differ only in the way in which the wire leads are connected to the body of the resistor. Both types use the same color code, but the colors are painted in a differ- ent manner on each type. Radial lead resistors are constructed with the wire leads wound around the ends of the carbon rod which makes up the body of the resistor. The leads come off at right angles and the entire resistor body — including the leads wound around the body — is painted but not insulated, since the paint is not a good insulator. Because of this poor insulation, this type of resistor must be mounted where it will not come into contact with other parts of a circuit. Radial lead resistors are rarely found in modern equipment al- though they were widely used in the past. Axial lead resistors are made with the leads molded into the ends of the carbon rod of the resistor body. The leads extend straight out from the ends and in line with the body of the resistor. The carbon rod is com- pletely coated with a ceramic material which is a good insulator. 1-113 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistor Color Code (continued) As mentioned on the previous sheet, the radial lead type and the axial lead type resistors both use the same color code, but the colors are painted on in a different manner for each of the two types. Radial resis- tors are coded with the body-end-dot system — as are a few axial type re- sistors. Most axial resistors are coded by the end-to-center band system of marking. In each color code system of marking, three colors are used to indicate the resistance value in ohms, and a fourth color is sometimes used to in- dicate the tolerance of the resistor. By reading the colors in the correct order and by substituting numbers from the color code, you can immedi- ately tell all you "need to know about a resistor. As you practice using the color code shown on the next sheet, you will soon get to know the numer- ical value of each color and you will be able to tell the value of a resis- Before you go on to the color code, you should find out something about resistor tolerance. It is very difficult to manufacture a resistor to the exact value required. For many uses the actual resistance in ohms can be 20 percent higher or lower than the value marked on the resistor with- out causing any difficulty. Many times the actual - resistance required need be no closer than 10 percent higher or lower than the marked value. This percentage variation between the marked value and the actual value of a resistor is known as the "tolerance" of a resistor. A resistor coded for a 5-percent tolerance will be no more than 5 percent higher or lower than the value indicated by the color code. 1-114 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistor Color Code (continued) This is how you use the color code — Color Number Tolerance Black 0 - Brown 1 1% Red 2 2% Orange 3 3% Yellow 4 4% Green 5 5% Blue 6 6% Color Number Tolerance V iolet 7 7% Gray 8 8% White 9 9% Gold - 5% Silver - 10% No Color - 20% Body-End-Dot Marking Resistors using this system of marking are coded by having the body of the resistor a solid color, one end of another color and a dot of a third color near the middle of the resistor. For example, you may have a re- sistor with a green body, red end and orange dot. The body color indi- cates the first digit, the end color the second digit and the dot the number of zeros to be added to the digits. The value of the resistor then is 52,000 ohms, obtained as follows— Body End Dot 1st digit 2nd digit Green Red 5 2 I 52,000 ohms Number of zeros Orange 000 Axial resistors are usually marked with bands of color at one end of the resistor. The body color is not used to indicate the resistor value and may be any color that is not identical to any of the color bands. For ex- ample, you may have a resistor with a brown body, having three bands of color (red, green and yellow) at one end. The color bands are read from the end toward the center and the resistor value is 250,000 ohms, ob- tained as follows — 1st Band 2nd Band 3rd Band 1st digit 2nd digit Number of zeros Red Green Yellow 2 5 0000 250,000 ohms 1-115 WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistor Color Code (continued) Whenever the center dot or the third band are black the resistor value is less than 100 ohms, since black means that no zeros are to be added to the digits. Suppose you have two resistors — one with a brown body, green end and black dot, the other with a red band, an orange band and a black band. You read these resistor values and find that they are 15 ohms and 23 ohms, obtained as follows — Body Brown End Green Dot Black 1st Band Red ft 2nd Band Orange o 3rd Band Black 1 J j | 15 ohms 23 ohms If the same color is used more than once, the body, end and dot may all be the same color or any two may be the same; but the color code is used in exactly the same way as before. For example, a 33,000-ohm resistor will be entirely orange if the body-end-dot marking is used, or will have three orange bands if the end-to-center marking is used. Body End Dot 1st Band 2nd Band 3rd Band Orange Orange Orange Orange Orange Orange 3 3 000 3 3 000 33,000 ohms 33,000 ohms WHAT CONTROLS CURRENT FLOW— RESISTANCE Resistor Color Code (continued) If only three colors are used, the tolerance (accuracy) of the coded value is 20 percent; but, if a fourth color is used, it indicates that the tolerance is less than 20 percent as indicated by the color code. A silver dot any place on the resistor indicates a tolerance of 10 percent while a gold dot indicates a tolerance of 5 percent. A fourth band on axial resistors is used to indicate tolerance and, if it is a color other than silver or gold, the tolerance in percentage corresponds to the number assigned that color in the color code. RESISTOR MARKINGS No tolerance marking 4th Indicates tolerance BAND listed in color chart Carbon resistors are made in values which have only two significant fig- ures followed by zeros. Resistors are available between 1 and 99 ohms differing in value by only 1 ohm — for example, 55 and 56 ohms; but be- tween 100 and 1,000 ohms the difference is 10 ohms — for example, 550 and 560 ohms. Similarly between 1,000 and 10,000 the nearest values differ by 100 ohms, and between 100,000 and 1,000,000 the nearest values differ by 10,000 ohms. If you need a value which is not obtainable — for example, 5,650 ohms — two resistors in series are used. To obtain 5,650 ohms you can use several combinations: 5,600 and 50 ohms, 5,000 and 650 ohms, 5,200 and 450 ohms, etc. However, for most of your work in electricity the closest value obtainable is used since accuracy beyond two digits is not normally required. 1-117 WHAT CONTROLS CURRENT FLOW— RESISTANCE Demonstration— the Ohmmeter To demonstrate the correct use of the ohmmeter for measuring resistance the instructor next shows how to operate and use the multirange ohmmeter for measuring resistance. During this demonstration you see that the in- structor uses only the ohmmeter ranges— R, R x 10, R x 100 and R x 1000- turning the RANGE SELECTOR SWITCH to one of these ranges before in- serting the test leads. With the test leads inserted in the meter jacks marked RES OHMS, the instructor touches the test prods together to find out if the meter deflects to approximately full scale. The range selector switch is set at the de- sired range and the OHMMETER ADJUSTER control is adjusted to obtain exactly full-scale deflection — zero ohms on the meter scale. To measure a resistor, the instructor "zeros" the meter after selecting the correct range and then touches the test prods to the two leads of the resistor. The meter will then indicate a resistance reading. If the range used is R the resistance is read directly on the top scale of the meter, but should one of the other ranges be used the scale reading is multiplied by the multiplier for that range. For example, if the meter is set to the range R x 100 and the meter scale reading is 50, the resistance is 5,000 ohms. 1-118 WHAT CONTROLS CURRENT FLOW— RESISTANCE Demonstration — the Ohmmeter (continued) As several resistors are measured, observe that each time the meter range is changed it must be "zeroed" again, as the zero adjustment is slightly different for each range. While the instructor measures those re- sistors which you have previously checked with the color code, compare the measured values to those you obtained at that time. Allowing for the tolerance rating of the resistors, the values should be about the same, but in most cases the meter reading is more accurate. While the instructor measures the various resistors, you can see the im- portance of choosing the correct meter range. Low values of resistance read 0 ohms on the higher ohmmeter ranges, and high values of resist- ance read the maximum scale reading on the lower ohmmeter ranges. For example, a 100-ohm resistor measured on the R x 10,000 scale reads 0 ohms, and a 10,000-ohm resistor measured on the R scale reads infinite ohms. In order to find the correct ohmmeter range to use, the best pro- cedure, as illustrated by the instructor, is to place the test prods on the resistor leads and turn the ohmmeter range switch through all the ranges until a range is found which gives a reading near mid-scale. Remember though, before an accurate reading can be made, the ohmmeter must be "zeroed" on the range being used. Next the instructor connects one meter test prod to the center terminal of a variable resistor and the other test prod to one of the outside terminals of the variable resistor. To show how resistance can be varied he turns the shaft, and you see that the resistance between these terminals changes as the shaft turns. With the meter leads connected across the outside terminals of the variable resistor, the shaft is again turned and you see that the resistance between these terminals is not varied. WHAT CONTROLS CURRENT FLOW— RESISTANCE Demonstration — Resistance Factors You have seen the different resistors measured, and perhaps you have wondered how resistors of identical size and shape can have such a range of resistance values. In carbon resistors the carbon rod is made of finely ground graphite (carbon) mixed with a filler material, and by varying the amount of carbon used in the mixture the resistance is varied over a wide range. For wire resistors the resistance is changed by using a different size or length of wire and by using wires of different materials, but using the same size porcelain or bakelite forms. To show the effect of the type material on the resistance of a conductor the instructor takes two equal lengths of wire — one of copper pushback wire and the other of nichrome — and measures their resistance. Notice that the copper wire has less than 1 ohm of resistance while the nichrome wire has more than 1 ohm of resistance. Using nichrome wire, the instructor next demonstrates the effect of length and cross-section on resistance. To show the effect of conductor length on resistance, two lengths of wire are used, one being twice as long as the other. Using the ohmmeter, the resistance of each wire is measured, and you see that the longer wire has twice the resistance of the other. The longer wire is then bent double and twisted to form one length equal to the length of the short wire, but having twice the cross-section. Now when the resistances are measured you see that the length of doubled wire has the lower resistance because of its greater cross-section. Wires having greater cross-section not only have lower resistance but also can carry more current, since more paths are available for current flow. You will find out more about the effects of increased cross-section later, when you work with parallel circuits. Single cross-section Double cross-section Nichrome wire Nichrome wire 1-120 WHAT CONTROLS CURRENT FLOW— RESISTANCE Review of Resistance Now you are ready to perform an experiment on resistors, but first briefly review what you have read and seen concerning resistance and how it is measured. CONDUCTOR — A material which gives up "free" electrons easily and offers little op- position to current flow. INSULATOR — A material which does not give up "free" electrons easily and offers great opposition to current flow. RESISTANCE — Opposition offered by a material to the flow of current. OHM — Basic unit of resistance measure equal to that resistance which allows 1 am- pere of current to flow when an emfof 1 volt is applied across the resistance. MEGOHM -- One megohm equals one million ohms. MICROHM — One microhm equals one -millionth ohm. 1 Meg A = 1,000,000 n 1 K /l = 1,000,000 rL OHM ME TER -- Meter used to meas- ure resistance directly. RESISTOR -- Device having resist- ance used to control current flow. 1-121 REVIEW Review of Current, Voltage and Resistance As a conclusion to your study of electricity in action you should consider again what you have found out about current, voltage and resistance. \ V CURRENT — Movement of free electrons through a conductor from a more nega- tive charge to the more positive charge. © 01*14 1 I RESISTANCE — The opposition a material offers to the flow of current. Particularly you should recall the relationships between current, voltage and resistance. Current flow is caused by the voltage between two points and is limited by the resistance between the points. In continuing your study you will next find out about electric circuits and how they use cur- rent, voltage and resistance. 1-122 INTRODUCING OHM’S LAW The Relationship Between Voltage, Current and Resistance Voltage, as you know, is the amount of electromotive force (emf) that is ap- plied across a load (resistance) in order to make an electron current flow through the resistance. It should be easy for you to see that the greater the voltage you apply across a resistance the greater will be the number of electrons that flow through in a second. Similarly, the lower the voltage you apply, the smaller will be the electron current. Resistance, as you know, is the effect that impedes the flow of electrons. If you increase the resistance of the load across' which a constant voltage is applied, less electron current will flow. Similarly, the lower you make the resistance, the greater will be the electron flow. This relationship between voltage, resistance and current as described in the two previous paragraphs was studied by the German mathematician George Simon Ohm. His description, now known as Ohm's law, says that current varies directly with the voltage and inversely with the resistance. The mathematical analysis of this law is of no concern to you at present, but you will learn about it when you get into Volume 2 INTRODUCING OHM’S LAW Electron Current Flow Low Voltage - Small Current + Low Resistance - Large Current + High Voltage - Large Current High Resistance - Small Current 1 -J- — With Constant Resistance With Constant Voltage 1-123 INDEX TO VOL. 1 (Note: A cumulative index covering all five volumes in this series will be found at the end of Volume 5.) Atom, 1-7 Charges, electric, 1-10 to 1-17, 1-60 Chemical action, electricity from, 1-23 Color code, resistor, 1-1 13 to 1-117 Conductors, 1-101 Current flow, 1-42 to 1-50 direction of, 1 -49 measuring, 1-61 to 1-63 Demonstration, Ammeter Ranges, 1-70, 1-71 Magnetic Fields, 1-37 Magnetic Fields Around a Conductor, 1-56 to 1-58 Ohmmeter, 1-118, 1-119 Range Selection and Correct Voltmeter Connection, 1-96 Reading Meter Scales, 1-72 Resistance Factors, 1-120 Voltage and Current Flow, 1-92, 1-93 Voltmeter Ranges, 1-95 Dry cells and batteries, 1-26 Electromagnetism, 1-51 to 1-55 Electron theory, 1-1, 1-2 EMF, 1-83 to 1-85 Friction, static charges from, 1-1 1 Galvanometer, 1 -78 Heat, electric charges from, 1-20 Insulators, 1-101 Length, affecting resistance, 1-103 Light, electric charges from, 1-21, 1-22 Magnetic field of loop or coil, 1-53 Magnetism, 1-31 to 1-36 Material, affecting resistance, 1-102 Matter, 1-3, 1-4 Meter movement, basic, 1-74 to 1-79 Meter range, usable, 1-69 Meter ranges, changing, 1-80 Meter scales, reading, 1-67, 1-68 Milliammeter and microammeter, 1-66 Molecule, structure of, 1 -5, 1 -6 Photo cell, 1 -21 Pressure, electric charges from, 1-19 Primary cell, 1-24, 1-25 Resistance, 1-98 to 1-100 factors controlling, 1-102 measurement of, 1-108 units of, 1-106, 1-107 Resistors, construction and properties, 1-109 to 1-111 Review, Current Flow, 1-50 Current, Voltage and Resistance, 1-122 Electricity and How It Is Produced, 1-41 Electricity — What It Is, 1-8 Electromagnetism, 1-59 Friction and Static Electric Charges, 1-18 How Current Is Measured, 1-73 Meter Movement, 1-82 Resistance, 1-121 Voltage Units and Measurement, 1-97 Secondary cell, 1-27, 1-28 Storage batteries, 1-29 Temperature, affecting resistance, 1-105 Voltage and current flow, 1-86, 1-87 Voltage, units of, 1-88 Voltmeter, 1-90, 1-91 multi-range, 1-94 1-124 HOW THIS OUTSTANDING COURSE WAS DEVELOPED: In the Spring of 1951, the Chief of Naval Personnel, seeking a streamlined, more efficient method of presenting Basic Electricity and Basic Electronics to the thousands of students in Navy specialty schools, called on the graphio- logical engineering firm of Van Valkenburgh, Nooger & Neville, Inc., to prepare such a course. This organization, specialists in the production of complete “packaged training programs,” had broad experience serving in- dustrial organizations requiring mass-training techniques. These were the aims of the proposed project, which came to be known as the Common-Core program: to make Basic Electricity and Basic Electronics completely understandable to every Navy student, regardless of previous education; to enable the Navy to turn out trained technicians at a faster rate (cutting the cost of training as well as the time required), without sacrificing subject matter. The firm met with electronics experts, educators, officers-in -charge of various Navy schools and, with the Chief of Naval Personnel, created a dynamic new training course . . . completely up-to-date . . . with heavy emphasis on the visual approach. First established in selected Navy schools in April, 1953, the training course comprising Basic Electricity and Basic Electronics was such a tremendous success that it is now the backbone of the Navy’s current electricity and electronics training program!* The course presents one fundamental topic at a time, taken up in the order of need, rendered absolutely understandable, and hammered home by the use of clear, cartoon-type illustrations. These illustrations are the most effec- tive ever presented. Every page has at least one such illustration — every page covers one complete idea! An imaginary instructor stands figuratively at the reader’s elbow, doing demonstrations that make it easier to understand each subject presented in the course. Now, for the first time, Basic Electricity and Basic Electronics have been released by the Navy for civilian use. While the course was originally de- signed for the Navy, the concepts are so broad, the presentation so clear — without reference to specific Navy equipment — that it is ideal for use by schools, industrial training programs, or home study. There is no finer training material! *“ Basic Electronics.” the second portion of this course, is available as a separate series of volumes. JOHN F. RIDER PUBLISHER, INC, 116 WEST 14th ST., N. Y. 11, N. Y. No. 169-2 basic electricity VOL. % by VAN VALKENBURGH, NOOGER & NEVILLE, INC. DIRECT CURRENT CIRCUITS OHM’S & KIRCHHOFF’S LAWS ELECTRIC POWER a RIDER publication $ 2.25 basic electricity by VAN VALKENBURGH, NOOGER A NEVILLE, INC. VOL. 1 JOHN F. RIDER PUBLISHER, INC. 116 W.«t 14 tfc Stmt • New Y.rk 11, N. Y. First Edition Copyright 1954 by VAN VALKENBURGH, NOOGER AND NEVILLE, INC. All Rights Reserved under International and Pan American Conventions. This book or parts thereof may not be reproduced in any form or in any language without permission of the copyright owner. Library of Co)igress Catalog Card No. 54-12946 Printed in the United States of America PREFACE The texts of the entire Basic Electricity and Basic Electronics courses, as currently taught at Navy specialty schools, have now been released by the Navy for civilian use. This educational program has been an unqualified success. Since April, 1953, when it was first installed, over 25,000 Navy trainees have benefited by this instruc- tion and the results have been outstanding. The unique simplification of an ordinarily complex subject, the exceptional clarity of illustrations and text, and the plan of pre- senting one basic concept at a time, without involving complicated mathematics, all combine in making this course a better and quicker way to teach and learn basic electricity and electronics. The Basic Electronics portion of this course will be available as a separate series of volumes. In releasing this material to the general public, the Navy hopes to provide the means for creating a nation-wide pool of pre-trained technicians, upon whom the Armed Forces could call in time of national emergency, without the need for precious weeks and months of schooling. Perhaps of greater importance is the Navy’s hope that through the telease of this course, a direct contribution will be made toward increasing the technical knowledge of men and women throughout the country, as a step in making and keeping America strong. Van Valkenburgh, Nooger and Neville , Inc. New York , N. Y. October , 1954 iii TABLE OF CONTENTS Vol. 2 — Basic Electricity What A Circuit Is 2-1 Direct Current Series Circuits 2-7 Ohm’s Law 2-24 Electric Power 2-42 Direct Current Parallel Circuits 2-55 Ohm’s Law and Parallel Circuits 2-73 Direct Current Series -Parallel Circuits 2-90 Kirchhoff’s Laws 2-103 V BASIC ELECTRICITY Qjt’tet'/ cfiVient crtr/ffh viii WHAT A CIRCUIT IS Electric Circuits Wherever two charges are connected by a conductor, a pathway for current flow exists; and if the charges are unequal, current flows from the nega- tive to the positive charge. The amount of current flow depends on the voltage difference of the charges and the resistance of the conductor. If two charged bars are connected by a copper wire, for example, current will flow from the more negative to the more positive bar, but only long enough to cause eachrbar to have an equal charge. Although current flows briefly, this kind of connection is not an electrical circuit. An electric circuit is a completed electrical pathway, consisting not only of the conductor in which the current flows from the negative to the posi- tive charge, but also of a path through a voltage source from the positive charge back to the negative charge. As an example, a lamp connected across a dry cell forms a simple electric circuit. Current flows from the (-) terminal of the battery through the lamp to the (+) battery terminal, and continues by going through the battery from the (+) to the (-) terminal. As long as this pathway is unbroken.it is a closed circuit and current flows; but, if the path is broken at any point, it is an open circuit and no current flows. 2-1 WHAT A CIRCUIT IS Electric Circuits (continued) A closed loop of wire is not always a circuit. Only if a source of emf is part of the loop do you have an electric circuit. In any electric circuit where electrons move around a closed loop, current, voltage and resist- ance are present. The pathway for current flow is actually the circuit, and its resistance controls the amount of current flow around the circuit. Direct current circuits consist of a source of DC voltage, such as batteries, plus the combined resistance of the electrical equipment connected across this voltage. While working with DC circuits, you will find out how the total resistance of a circuit is changed by using various combinations of re- sistances, how these combinations control the circuit current and affect the voltage. You have found out how cells are connected in series or parallel, and now you will find that resistances are connected in the same manner to form the two basic types of circuits, series and parallel circuits. No matter how complex a circuit you may work with, it can always be broken down into either a series circuit connection or a parallel circuit connection. Ammeter WHAT A CIRCUIT IS Simple Circuit Connections Only the resistances in the external circuit, between the terminals of the voltage source, are used to determine the type of circuit. When you have a circuit consisting of only one device having resistance, a voltage source and the connecting wires, it is called a SIMPLE circuit. For example, a lamp connected directly across the terminals of a dry cell forms a simple circuit. Similarly, if you connect a resistor directly across the terminals of a dry cell, you have a simple circuit since only one device having re- sistance is being used. Simple circuits may have other devices connected in series with a lamp but the nature of the circuit does not change unless more than one resist- ance is used. A switch and a meter inserted in series with the lamp do not change the type of circuit since they have negligible resistance. simple circuit Whenever you use more than one device having resistance in the same cir- cuit, they will be connected to form either a SERIES or PARALLEL cir- cuit, or a combination SERIES-PARALLEL circuit. 2-3 WHAT A CIRCUIT IS Switches You already know that, in order for current to flow through a circuit, a closed path must be provided between the + and - terminals of the voltage source. Any break in the closed path opens the circuit and stops the flow of current. CURRENT FLOW REQUIRES A CLOSED PATH Current flow through No closed path— a closed path no current flow Until now we have stopped current flow by removing a battery lead. Since this is not a suitable method for opening a practical circuit, switches are actually used. A CIRCUIT MAY BE OPENED BY: Removing a battery lead or ... . opening a switch WHAT A CIRCUIT IS Switches (continued) A switch is a device used to open and close a circuit or part of a circuit when desired. You have been using switches all your life— in lamps, flash- lights, radio, car ignition, etc. You will meet many other kinds of switches while working with equipment. Knife switch Potentiometer switch jd»>| Toggle switch House light switch In the demonstrations and experiments to follow, a switch will be inserted in one of the battery leads. You will use a "single-pole, single-throw knife switch," which looks like this: and is represented symbolically like this: Closed Open 2-5 WHAT A CIRCUIT IS Circuit Symbols Electrical circuit connections are usually shown in symbol form in the same manner as the dry cell and battery symbols which you have used pre- viously. You will find that symbols are not only used to represent various types of equipment and show circuit connections, but are also used to ex- press current, voltage and resistance. To express the amounts of current, voltage, resistance and power, the fol- lowing symbols are commonly used: voltage | Z current |^Z resistance pz power yz volts J^Z amperes ^Z ohms ^Z watts For example, in a simple circuit consisting of a lamp connected across a dry cell the voltage, current and resistance would be expressed as shown: E=1.5V (volts) 1=0.3 A (ampere) R=5n (ohms) P=0.45W (watt) (Circuit Scfm&oU fan 7£e4i&t(n& — wwwv— -vwwvv^ POTENTIOMETER DIRECT CURRENT SERIES CIRCUITS Series Circuit Connections Whenever you connect resistances end to end, they are said to be series- connected. If all the resistances around a circuit are connected end to end so that 'there is only one path for current flow, they form a series circuit. Tou have already found out how to connect cells in series to form a battery. An important difference between cells and resistances connected in series is that cells must be connected with the proper polarity but resistances are not polarized. Suppose you should connect a terminal of one lamp socket to a terminal on another socket, leaving one terminal on each socket unconnected. Lamps placed in these sockets would be series-connected, but you would not have a series circuit. To complete your series circuit, you would have to con- nect the lamps across a voltage source, such as a battery, using the un- connected terminals to complete the circuit. Any number of lamps, re- sistors or other devices having resistance can be used to form a series circuit, provided they are connected end to end across the terminals of a voltage source with only one path for current flow between these terminals. 2-7 DIRECT CURRENT SERIES CIRCUITS Resistances in Series One of the factors of resistance is length, with the resistance of a conduc- tor increasing as the conductor length increases. If you add one length of wire to another, the resistance of the entire length of wire is equal to the sum of the resistances of the original lengths. For example, if two lengths of wire — one having a resistance of 4 ohms and the other of 5 ohms — are connected together, the total resistance between the unconnected ends is 9 ohms. Similarly, when other types of resistances are connected in series, the total resistance equals the sum of the individ- ual resistances. OHMS IN SERIES, RESISTANCES ADD 4 OHMS -5 OHMS Whenever you use more than one of the same device or quantity in an elec- trical circuit, some method of identifying each individual device or quan- tity is necessary. For example, if three resistors of different values are used in a series circuit, something other than just R is needed to distin- guish each resistor. A system of identification called "subscripts" is used and consists of following the symbol of the device or quantity by a very small identification number. Ri, R2 and R3 are all symbols for resistors but each identifies a particular resistor. Similarly El, E2 and E3 are all different values of voltage used in the same circuit, with the small sub- script number identifying the particular voltage. SUBSCRIPTS IDENTIFY CURRENTS, VOLTAGES AND RESISTANCES 1 DIRECT CURRENT SERIES CIRCUITS Resistances in Series (continued) While numbers are used to identify individual electrical devices or quan- tities, a small letter "t" following the symbol indicates the total amount. You have found that when resistances are connected in series the total re- sistance equals the sum of the individual resistances. This might be ex- pressed as Rt = Ri + R2 + R3 where Rj, R2 and R3 represent resistors. The symbol R is also used to represent the resistance of other electri- cal devices. You will find that, although subscripts are one method of identifying indi- vidual devices or quantities, other methods are also used. Some of these other methods are shown below and compared to the subscript method of marking. Regardless of the method used, the marking serves only one purpose— identification of individual devices or quantities— and does not indicate a value. OTHER MARKINGS USED FOR IDENTIFICATION DIRECT CURRENT SERIES CIRCUITS Current Flow in Series Circuits In a series circuit there is only one path for current flow. This means that all the current must flow through each resistance in the circuit. All parts of the circuit then must be able to pass the maximum current which flows, and the total resistance of the circuit must be large enough to re- duce the amount of current to a value which can be safely passed by all the circuit resistances. Ammeters placed at each end of all the resistances of a series circuit would read the same amount of current flow through each resistance. In a circuit containing devices such as lamps in series, each lamp for proper operation should be rated with the same amount of current. Lamps rated to operate at higher currents than the circuit current will light only dimly, while lamps rated for less than the circuit current will light very brightly, and perhaps even burn out due to the excess current. The same effect would be noticed if the circuit contained other types of resistances. DIRECT CURRENT SERIES CIRCUITS Voltage in Series Circuits Whenever a force is exerted to move something against some form of op- position, the force is expended. For example, a hammer striking a nail exerts a force which moves the nail against the opposition offered by the wood, and as the nail moves, the force exerted is expended. Similarly, as emf moves electrons through a resistor, the force is expended, resulting in a loss of emf called "voltage drop." Starting at one end of a series circuit consisting of three resistors of equal value connected across a six-volt battery, the potential drops will be two volts across resistor Rj, four volts across Rj and R 2 , and six volts across ^1» ^2> R3> the entire circuit. The voltage across each resistor is two volts, and adding the voltages across the three resistors results in the original total voltage of six volts. This drop in voltage occurs because the current flow in a series circuit is always the same throughout the circuit. 2-11 DIRECT CURRENT SERIES CIRCUITS Demonstration — Series Circuit Resistance To demonstrate the effect of connecting resistances in series, the instruc- tor will measure the resistance of three lamps individually and then meas- ure their resistance in series. First the instructor connects three lamp sockets in series and inserts 6- volt lamps in the sockets. He then uses the ohmmeter to measure the re- sistance of each lamp, and you see that each lamp measures about one ohm. L2 DIRECT CURRENT SERIES CIRCUITS Demonstration— Series Circuit Resistance (continued) Using four dry cells connected in series to form a six-volt battery as a voltage source, the instructor demonstrates the effect of adding resistance in series. The voltmeter is connected across the battery and reads six volts, while the ammeter is connected in series with the negative lead of the battery to show the amount of current flow from the battery. With the ammeter in series, a single lamp socket is connected across the battery and a 6-volt lamp is inserted in the socket. You see that the lamp lights to normal brilliancy and that the ammeter reading is about 0.5 ampere. As the instructor moves the voltmeter to connect it directly across the lamp, you see that the voltage across the lamp is 6 volts. 2-13 DIRECT CURRENT SERIES CIRCUITS Demonstration — Series Circuit Resistance (continued) Next, the single lamp socket is replaced by three sockets in series and 6- volt lamps are inserted in the sockets. The lamps now light at well below normal brilliance, and the ammeter reading is about one -third its previous value. A voltmeter reading taken across the total circuit reads 6 volts and across each lamp the voltage is 2 volts. Since the voltage from the battery is not changed but the current decreased, the resistance must be greater. Adding the voltage across each lamp shows that the sum of the voltages across the individual resistances equals the total voltage. DIRECT CURRENT SERIES CIRCUITS Demonstration — Series Circuit Current To show the effect of changing resistances on the amount of current flow and how different equipment requires different amounts of current for proper operation, one of the 6-volt lamps is replaced by a 2.5-volt lamp of less resistance. You see that the two 6-volt lamps increase to al- most half of normal brilliancy while the 2. 5 -volt lamp is dim. The am- meter reading shows that the current increases, indicating that decreasing the resistance of one part of the circuit decreases ihe total opposition to current flow. Replacing another of the 6-volt lamps with a 2.5-volt lamp further decreases the total resistance and increases the total cir- cuit current. The brilliance of the lamps increases as the current flow increases, and with the last 6-volt lamp replaced by a lower resistance 2. 5 -volt lamp, you see that the circuit current for the three 2. 5 -volt lamps is approximately the same as that of a single 6 -volt lamp. Also, the three lamps light at about normal brilliancy because the current is only slightly less than the rated value of these lamps, as is the voltage measured across each lamp. 2-15 DIRECT CURRENT SERIES CIRCUITS Demonstration — Series Circuit Voltage The rated voltage of three 2.5-volt lamps in series is 7.5 volts so that the 6-volt battery does not cause the rated current to flow. By adding one more dry cell, the instructor shows how increasing the circuit voltage without changing the resistance will cause a greater current flow as vou can see by the increased brilliancy of the lamps and the increased current reading. Voltage readings taken across the lamps show that the voltage across each lamp is the rated voltage of 2.5 volts. Removing one cell of the battery at a time and taking voltage readings across the lamps, the in- structor shows that the voltages across the lamps equal each other and always add to equal the total battery voltage. Now using five cells to form a 7.5-volt battery, the instructor replaces one of the 2.5-volt lamps with a 6-volt lamp having greater resistance. Volt- meter readings across the lamps still total 7.5 volts when added, but are not all equal. The voltages across the lower resistance 2. 5 -volt lamps are equal but less than 2.5 volts, while the voltage across the higher resist- ance 6-volt lamp is greater than 2.5 volts. You can see that for resistors in series the voltage divides in proportion across the various resistances connected in series, with more voltage drop across the larger resistance and less voltage drop across the smaller resistance. 2-16 DIRECT CURRENT SERIES CIRCUITS Demonstration — Open Circuits You already know that in order for a current to pass through a circuit a closed path is required. Any break in the closed path causes an "open" circuit, and stops current flow. Each time you open a switch, you are causing an open circuit. Anything which causes an "open" other than actually opening a switch in- terferes with the proper operation of the circuit, and must be corrected. An open circuit may be caused by a loose connection, a burned-out re- sistor or lamp filament, poor joints or loose contacts, or broken wire. OPEN CIRCUITS can be caused by These faults can often be detected visually, and you may find that, in performing the experiments to follow, you 'will encounter one or more of these "opens." In some cases it is not possible to visually detect the cause of an open cir- cuit. The instructor will demonstrate how to use the ohmmeter or a test lamp to find the cause of trouble. 2-17 DIRECT CURRENT SERIES CIRCUITS Demonstration— Open Circuits (continued) The instructor connects five dry cells, a knife switch and three lamp sockets in series. He then inserts 2.5-volt lamps in the sockets. When he closes the switch the lamps light with normal brilliancy. He then loosens one of the lamps and they all go out, indicating an open circuit. (A loosened lamp simulates a burned-out filament or other open.) Creating an 1 ‘OPEN" To locate the open with the ohmmeter the instructor first opens the knife switch to remove the voltage source, since an ohmmeter must never be used on a circuit with the power connected. He then touches the ohm- meter test leads across each unit in the circuit — the three lamps in this case. You see that for two of the lamps the ohmmeter indicates a re- sistance under 10 ohms, but for the loosened lamp the ohmmeter indi- cates infinity. Since an open does not allow any current to flow, its re- sistance must be infinite. The ohmmeter check for an open, then, is to find the series-connected element in the circuit which measures infinite resistance on the ohmmeter. Good bulb — resistance less than 1011 "Open"— infinite resistance 2-18 DIRECT CURRENT SERIES CIRCUITS Demonstration — Open Circuits (continued) The second method used to locate an open is to test the circuit by means of a test lamp. The instructor 'attaches leads to the terminals of a lamp sock- et and inserts a 2. 5-volt lamp. He closes the circuit switch and touches the test lamp leads across each lamp in the circuit. The lamp does not light until he touches the terminals of the loosened lamp. The test lamp then lights, indicating he has found the open. The test lamp completes the circuit and allows current to flow, bypassing the open. You will use this method often to detect opens which cannot be seen. 2-19 DIRECT CURRENT SERIES CIRCUITS Demonstration — Short Circuits You have seen how an open prevents current flow by breaking the closed path between terminals of the voltage source. Now you will see how a "short" produces just the opposite effect, creating a "short circuit" path of low resistance through which a larger than normal current flows. A short occurs whenever the resistance of a circuit or part of a circuit drops from its normal value to zero resistance. This happens if the two terminals of a resistance in a circuit are directly connected; the voltage source leads contact each other, two current-carrying uninsulated wires touch, or the circuit is improperly wired. These shorts are called "external shorts" and can usually be detected by visual inspection. 2-20 DIRECT CURRENT SERIES CIRCUITS Demonstration — Short Circuits (continued) When a short occurs in a simple circuit, the resistance of the circuit to current flow becomes zero, so that a very large current flows. The effect of a on current flow In a series circuit, a short across one or more parts of the circuit results in reduction of the total resistance of the circuit and corresponding in- creased current, which may damage the other equipment in the circuit. Suited (Circuit RESULTS IN GREATER THAN NORMAL CURRENT rW NORMAL CIRCUIT \\\l Normal current ''MV vV Circuits are usually protected against excessive current flow by the use of fuses, which you will learn about later. But it is important that you un- derstand the reasons for and results of shorts so that you can avoid acci- dentally shorting your circuits and causing damage to meters or other equipment. 2-21 DIRECT CURRENT SERIES CIRCUITS Demonstration — Short Circuits (continued) The instructor connects three dry cells in series with a 0-1 amp range am- meter and three lamp sockets. He inserts 2.5-volt lamps in the sockets and closes the switch. You see that the lamps light equally but are dim, and the ammeter indicates a current flow of about 0.5 amp. A NORMAL SERIES CIRCUIT of one of the lamps, "short-circuiting" the current around that lamp. You see that the lamp goes out, the other two lamps become brighter, and the ammeter shows that the current has increased to about 0.6 amp. When he moves the lead to short out two of the lamps, you see that they both go out, the third lamp becomes very bright, and the current increases to about 0.9 amp. Since the lamp is rated at only 0.5 amp, this excessive current would soon burn out the lamp filament. SEEING THE EFFECT OF A SHORT IN A SERIES CIRCUIT If the instructor were to short out all three lamps, the lack of resistance of the circuit would cause a very great current to flow which would damage the ammeter. 2-22 DIRECT CURRENT SERIES CIRCUITS Review of Series Circuit Connections Consider now what you have found out so far about electric circuits and particularly series circuits. While a complete electric circuit always con- sists of a complete path for current flow through the voltage source and across the terminals of the voltage source, you have discovered that— for all practical purposes— the path through the voltage source is disregarded in considering a circuit. Only the connections and effect of the resistances connected across the terminals of the voltage source are considered. I Simple Circuit — A single resistance connect- ed across a voltage source. 1 Se ries C ircuit— Resistances connected end to end across a voltage source. Series Circuit Resistance — The total resist- ance equals the sum of the individual resist- ances. Series Circuit Current— The current is the same through' alf parts of the circuit. Ser ies ^lrcuitJVoltage— The voltage across each resistance is only a part of the total volt- age and depends on the value of each resistor. Each of these parts of the total voltage is called IR or VOLTAGE DROP, the sum of which equal the total or applied voltage. LAMP LAM Pi 0.5-AMP LAMP I Normal bri^htne: 3-AMP LAMP ■SI Verv bright HPPI 2-23 OHM'S LAW Ohm's Law in Simple Circuits You have found out that voltage and resistance affect the current flow in a circuit, and that voltage drops across a resistance. The basic relationships of current, voltage and resistance are as follows: 1. Current in a circuit increases when the voltage is increased for the same resistance. 2. Current in a circuit decreases when the resistance is increased for the same voltage. These two relationships combined are Ohm's Law, the most basic law of electric circuits, usually stated as follows: THE CURRENT FLOWING IN A CIRCUIT CHANGES IN THE SAME DI- RECTION THAT THE VOLTAGE CHANGES, AND IN THE OPPOSITE DI- RECTION THAT THE RESISTANCE CHANGES. &unne*tt cJuxtiyeb... ...IN THE SAME DIRECTION THAT THE VOLTAGE CHANGES. IN THE OPPOSITE DIRECTION THAT THE RESISTANCE CHANGES 2-24 OHM’S LAW Ohm's Law in Simple Circuits (continued) You have seen that if a certain current of electricity flows in a circuit, it flows because a certain electromotive force, or voltage, forces it to flow, and that the amount of current is limited by the resistance of the circuit. In fact, the amount of current depends upon the amount of electrical pres- sure, or voltage, and the amount of resistance. This fact was discovered by a man named George S. Ohm and is expressed by the now famous Ohm's Law which is the fundamental equation of all electrical science. Since it was first stated in 1827, this law has been of outstanding importance in electrical calculation. One of the most common ways of expressing Ohm's Law is that THE CURRENT FLOWING IN A CIRCUIT IS DIRECTLY PRO- PORTIONAL TO THE APPLIED VOLTAGE, AND INVERSELY PROPOR- TIONAL TO THE RESISTANCE. When you put this word statement into a mathematical relationship you get or CURRENT » ELECTROMOTIVE FORCE (or VOLTAGE) RESISTANCE AMPERES * VOLTS OHMS Ohm's Law can also be written in two other forms. VOLTAGE ■ CURRENT x RESISTANCE or VOLTS * AMPERES x OHMS This enables you to find the voltage when you know the current and resistance. If you know the voltage and the current, you can find the resistance then by simply applying the following form of (Kim's Law. or RESISTANCE « VOLTAGE CURRENT OHMS » VOLTS AMPERES 2-25 OHM'S LAW Ohm's Law in Simple Circuits (continued) Ohm's law is used in electric circuits and parts of circuits to find the un- known quantity of current, voltage or resistance when any two of these quantities are known. In its basic form Ohm's law is used to find the cur- rent in a circuit if the voltage and resistance are known. To find the cur- rent through a resistance, the voltage across the resistance is divided by the resistance. Current (amperes) = idSal ki Tohms I In symbol form l= R As you know, the current in a circuit increases if the voltage increases and the resistance remains the same. By giving values to E and R, you can see how this works. Suppose that R is 10 ohms and E is 20 volts. Since the current equals 20 divided by 10, the current is 2 amperes as shown: Now if E is increased to 40 volts without changing the resistance, the cur- rent increases to 4 amperes. Similarly, if the voltage remains the same and the resistance is increased, the current decreases. Using the original values where E is 20 volts and R is 10 ohms, you found that the current is 2 amperes. If R is increased to 20 ohms without changing the voltage, the current decreases to 1 ampere. 2-26 OHM'S LAW Ohm's Law in Simple Circuits (continued) £ While I = g is the basic form of Ohm's law and is used to find current, by expressing the law in other forms, it may be used to obtain either E or R. To use Ohm's law to find the resistance when voltage and current are known, the voltage is divided by the current. Resistance » In symbol form: R = -f As an example, if the current through a lamp connected across a 6 -volt battery is 2 amperes, the resistance of the lamp is 3 ohms. «=t r ir l I 6 V |R R = -f = 3 ohms T f A third use for Ohm's law is to find the voltage when the current and re- sistance are known. To find the voltage across a resistance, the current is multiplied by the resistance. Voltage = Current x Resistance E = l*R In writing electrical laws as formulas, the multiplication sign is not nor- mally used, so that (Kim's law for voltage is expressed as: E = I R To find the voltage across a 5 -ohm resistor when 3 amperes of current are flowing, you must multiply I times R, so that the voltage equals 15 volts. E E In using Ohm's law, the quantities must always be expressed in the basic units of current, voltage and resistance. If a quantity is given in larger or smaller units, it must first be changed so that it is expressed in am- peres, volts or ohms. 2-27 OHM'S LAW Establishing Total Resistance in Series Circuits In the previous topic you learned that the total resistance in a series cir- cuit is equal to the sum of the individual resistances in that circuit. Total resistance in a series circuit, called Rip, may be established by using Ohm's Law if the amount of current in the circuit and the impressed volt- age are known. Consider the schematic diagram below. Note that the total impressed volt- age, E-p, is 100 volts, and that the total current in the circuit, I>p, is two amperes. Note also, that there are three resistors in series. This fact will not cause any difficulty in solving the problem if you remember that the total current flowing in a circuit is the result when the total voltage is applied across the total resistance in the circuit. Using Ohm's Law, then, the total resistance is equal to the total voltage divided by the total current. Applying Ohm's Law To A Series Circuit I Total = 2 A™? 61,68 E Total - 100 Volts R 1 *2 r 3 ^Total “ R 1 + r 2 + R 3 RTotal = R 1 + r 2 + r 3 Rrotal E Total *Total Rip 100 Volts 2 Amperes R>P = 50 Ohms In the previous topic you also learned that when the voltage drops in a se- ries circuit are added together, the total value is equal to the total im- pressed voltage, or E Total = E 1 + e 2 + e 3 You learned, too, that the current flowing in a series circuit is everywhere the same, or I Total = X 1 = x 2 * 13 This is true even though the various resistors in the series circuit may all be of different values. 2-28 OHM'S LAW Ohm's Law in Series Circuits You can use Ohm's law in working with series circuits, either as applied to the entire circuit or to only a part of the circuit. It can only be used to find an unknown quantity for a certain part of the circuit when two factors are known. Consider a circuit consisting of three resistors connected in series across 100 volts, with a circuit current flow of 2 amperes. If two of the resistor values, Ri and R2, are known to be 5 ohms and 10 ohms respectively, but the thira resistor value R3 is not known, the value of R3 and the current and voltage for each resistor may be determined by applying Ohm's law to each part of the circuit. To find the unknown values, you should first make a simple sketch, see the diagram below, for recording the information which you already have and that which you will obtain as you use Ohm's Law for various parts of the circuit. This sketch will enable you to visualize the various components of the circuit and their relationships with one another. Next you should record all of the known factors concerning each resistor. You know that Rj equals 5 ohms and R2 equals 10 ohms and also that the circuit current is 2 amperes. Since there is only one path for current in a series circuit, the current is the same in every part of the circuit and is equal to 2 amperes. 2-29 OHM'S LAW Ohm's Law in Series Circuits (continued) For Rj and R 2 you have two known quantities — resistances and currents — and can therefore find the voltages. Using Ohm's law to find the voltage across R U for example, the current — 2 amperes — is multiplied by the re- sistance — 5 ohms— resulting in a voltage of 10 volts across Rj. Similarly the voltage across Ro is found by multiplying the current by the resistance — 2 amperes timesJO ohms — resulting in a voltage of 20 volts across R 2 . FINDING E, AND E : 100 VOLTS l, = 2A Rj = 50 Ij = 2 amps Ej = 10 volts R) =5 imp® mwsBBSM I E|= 10 VOLTS It 2 =10 A 1 R2 = 100 12 = 2 amps 1 E 2 = 20 volts E 2 = 20 VOLTS h Rs- I 3 = 2 amps E 3 = Your sketch now is complete except for the resistance value and voltage across R 3 . If you can obtain the correct value of either the resistance or the voltage for R 3 , the other quantity can easily be found by applying Ohm's law to R 3 . 2-30 OHM'S LAW Ohm's law in Series Circuits (continued) Since the three resistors are connected across 100 volts, the voltages across the three resistors must equal 100 volts when added together. If the voltages across Rj and R2 are equal to 10 volts and 20 volts respec- tively, the total voltage across the two equals 30 volts. Then the voltage across R3 must equal the difference between the total 100 volts and the 30-volt total across Rj and R2, or 70 volts. Ohm's law can be used to find the resistance of R3 by dividing the voltage — 70 volts— "by the current — 2 amperes — so that R3 equals 35 ohms. 2-31 OHM'S LAW Ohm's Law in Series Circuits (continued) You can also use another method of finding the unknown quantities for Rg. Since the total circuit voltage and current is known, the total circuit re- sistance can be found by dividing the voltage — 100 volts — by the current—* 2 amperes. The total resistance then is 50 ohms and, since this total must equal the sum of Rj, Rg and Rg, the value of R3 is equal to the difference between 50 ohms and Ri plus Rg* The sum of Ri and R2 equals 15 ohms, leaving a difference ot 35 ohms as the resistance value of R3. With the resistance value and current for R3 known, the voltage is found by multi- plying the two known quantities. Multiplying the resistance 35 ohms by the current — 2 amperes — results in a voltage of 70 volts across R3. The results are the same as those previously obtained. 2-32 OHM'S LAW Ohm’s Law in Series Circuits (continued) With the values of R3 and E3 known, your table is now complete, giving all the values of resistance, voltage and current for each of the three resis- tors in the circuit. From the completed table of values you can find the total circuit resistance, voltage and current. Since the circuit is series- connected, the current for the total circuit is the same as that for any part of the circuit, while the total voltage and the total resistance are found by adding the individual voltages and resistances. (fauftleted ladle 'l/aluea Part of Circuit Resistance Voltage Current 5 Ohms 10 Volts 2 Amperes R2 10 Ohms 20 Volts 2 Amperes *3 35 Ohms 70 Volts 2 Amperes The total resistance (Rj) is equal to 50 ohms, the total current (It) is 2 amperes, and the total voltage (E^) is 100 volts. Now you know all of the circuit values. It = 2 Amperes t AA/WWV 4 i R.; 2 *2 and e 2 are used together. Only quantities having the same subscript can be used together to find an unknown by means of Ohm's law. Unknown quantities may also be found by applying the following rules for parallel circuits: DIRECT CURRENT SERIES-PARALLEL CIRCUITS Series-Parallel Circuit Connections Circuits consisting of three or more resistors may be connected In a com- plex circuit, partially series and partially parallel. There are two basic types of series-parallercircuits: One in which a resistance is connected in series with a parallel combination, and the other in which one or more branches of a parallel circuit consist of resistances in series. If you were to connect two lamps in parallel (side-by-side connection) and connect one terminal of a third lamp to one terminal of the parallel com- bination the three lamps would be connected in series -parallel. Resist- ances other than lamps may also be connected in the same manner to form series-parallel circuits. You can connect the three lamps to form another type of series-parallel circuit by first connecting two lamps in series, then connecting the two terminals of the third lamp across the series lamps. This forms a par- allel combination with one branch of the parallel circuit consisting of two lamps in series. Such combinations of resistance are frequently used in electrical cir- cuits, particularly electric motor circuits and control circuits lor electrical equipment. TWO WAYS OF CONNECTING LAMPS IN SERIES-PARALLEL r Series part of circuit 1 I / / / N 1 \ | \ [ { ii ■VW — ! t \ \ \ n / 1 / . ✓ I * u \ \ \ ■Wi: Parallel branches I of circuit l I / T - Series part of circuit i \ I \ Parallel branches of circuit 2-90 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Resistances in Series-Parallel No new formulas are needed to find the total resistance of resistances connected in series -parallel. Instead you break the complete circuit into parts consisting of simple series and parallel circuits, then solve each part separately and combine the parts. Before using the rules for series and parallel resistances, you must decide what steps to use in simplify- ing the circuit. For example, suppose you want to find the total resistance of three re- sistances— R^, R2 and R3 — connected in series-parallel, withRi and Ro connected in parallel, and R3 connected in series with the parallel com- bination. To simplify the circuit you would use two steps, with the circuit broken down into two parts— the parallel circuit of Ri and R 2 , and the series resistance R3. First you find the parallel resistance of Ri and Ro, using the formula for parallel resistances. This value is then added to the series resistance R3 to find the total resistance of the series- parallel circuit. If the series-parallel circuit consists of Ri and R 2 in series, with Ro con- nected across them, the steps are reversed. The circuit is broken down into two parts— the series circuit of Ri and R 2 , and the parallel resist- ance R3. First you find the series resistance of Ri and Ro by adding- then combine this value with R 3 , using the formula for parallel resistance. Combine Rj and R2 to find total resistance (R a ) of parallel combination Ra = _ RiX R R,+R 2 Add Ra and R3 to find total circuit resistance (Rt) Rt = Ra + Rj FINDING THE TOTAL iv ESiST -\N( E 1 ) E SERIES- PARA I, EEL CIRCUIT Add Rj and R2 to find total resistance (R a ) of series-connected branch Ra = R,+R 2 Combine the parallel combination of R a and R3 to find the total circuit resistance (Rt) «t= RaxR; Ra + Ri 2-91 DIRECT CURRENT SERIES -PARALLEL CIRCUITS Resistances in Series-Parallel (continued) Complex circuits may be simplified and their breakdown made easier by redrawing the circuits before applying the steps to combine resistances. THE ORIGINAL CIRCUIT 3. Where the parallel paths com- bine, a line is drawn across the ends to join the paths. 4. The circuit is continued from the center of the parallel con- necting line, adding the series resistance to complete the re- drawn circuit. 2-92 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Resistances in Series-Parallel (continued) The basic steps in finding the total resistance of a complex series- parallel circuit are as follows — 1. Redraw the circuit if necessary. 2. If any of the parallel combinations have branches consisting of two or more resistors in series, find the total value of these resistors by adding them. 3. Using the formula for parallel resistances, find the total resistance of the parallel parts of the circuit. 4. Add the combined parallel resistances to any resistances which are in series with them. DIRECT CURRENT SERIES-PARALLEL CIRCUITS Resistances in Series-Parallel (continued) This is how you break down complex circuits tofind the total resistances Suppose your circuit consists of four resistors ““Rl» ^2* r 3 “d R 4 — connected as shown, and you want to find the total resistance of the circuit. First, the circuit is redrawn and the series branch resistors R3 and R 4 are combined by addition as an equivalent resistance R a . Ra = R3 + R4 O Vw Rl r 2 i : R3 : 0 R4 W Next, the parallel c ombination, of R 2 and R a is comlsihM parallel resistance formula) as an equivalent resistance, R^. R b = R 2 xR a R 2 + R a The series resistor Ri is added to the equivalent resistance — Rju — of the parallel combination to find the total circuit resist- ance, Rt. R t = R i + Rb O Rf = total resistance of series- parallel circuit. Rt Rt = Total Resistance 0 2-94 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Resistances in Series -Parallel (continued) More complicated circuits only require more steps, not any additional formulas. For example, the total resistance of a circuit consisting of nine resistors may be found as shown — DIRECT CURRENT SERIES-PARALLEL CIRCUITS Current in Series -Parallel Circuits The total circuit current for a series-parallel circuit depends upon the total resistance offered by the circuit when connected across a voltage source. Current flow in the circuit will divide to flow through all parallel paths and come together again to flow through series parts of the circuit. It will divide to flow through a branch circuit and then repeat this division if the branch circuit subdivides into secondary branches. As in parallel circuits, the current through any branch resistance is in- versely proportional to the amounts of resistance— the greater current flows through the least resistance. However, all of the branch currents always add to equal the total circuit current. The total circuit current is the same at each end of a series-parallel cir- cuit and equals the current flow through the voltage source. 2-97 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Voltage in Series-Parallel Circuits Voltage drops across a series- parallel circuit occur in the same way as in series and parallel circuits. Across series parts of the circuit the voltage drops are equal only for equal resistances, while across parallel parts of the circuit the voltage across each branch is the same. Series resistances forming a branch of a parallel circuit will divide the voltage across the parallel circuit. In a parallel circuit consisting of a branch with a single resistance and a branch with two series resistances, the voltage across the single resistor equals the sum of the voltages across the two series resistances. The voltage across the entire parallel circuit is exactly the same as that across either of the branches. The voltage drops across the various paths between the two ends of the series-parallel circuit always add up to the total voltage applied to the circuit. 2-98 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Demonstration— Series-Parallel Connections Simple series-parallelcircuitconnections using three resistors are shown first. You see that three resistors having color code values of 30 ohms each are connected together, with one resistor in series with a parallel combination of the other two. This forms a series-parallel circuit and the total resistance is found by combining the parallel 30-ohm resistors to obtain their equivalent resistance, which is 15 ohms, and adding this value to the series 30-ohm resistor — making a total resistance of 45 ohms. You see that, with the resistors so connected, the ohmmeter reads 45 ohms across the entire circuit. Next, two resistors are connected in series and the third resistor is con- nected in parallel across the series combination. The total resistance is found by adding the two resistors in the series branch, to obtain the equiv- alent value of 60 ohms. This value is in parallelwith the third 30-ohm re- sistor, and combining them results in a value of 20 ohms for the total re- sistance. This value is checked with an ohmmeter and you see that the meter reading is 20 ohms. 2-99 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Demonstration — Current in Series-Parallel Circuits Next the instructor connects a series-parallel circuit — consisting of two 30-ohm resistors connected in parallel and a 15-ohm resistor in series with one end of the parallel resistors — across a six-volt dry cell battery. To show the path of current flow through the circuit, the instructor con- nects an ammeter in series with each resistor in turn — showing the cur- rent flow through each. You see that the current for the 15-ohm series resistor is 0.2 amperes as is the current at each battery terminal, while the current through the 30-ohm resistors is 0.1 amperes each. The circuit connections are changed with the 15-ohm and one 30-ohm re- sistor forming a series-connected branch in parallel with the other 30- ohm resistor. As the instructor connects the ammeter to read the various currents, you see that the battery current is 0.33 ampere, the 30-ohm re- sistor current is 0.2 ampere and the current through the series branch is 0.13 ampere. SEEING HOW THE CURRENT FLOWS THROUGH SERIES-PARALLEL CIRCUITS 2-100 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Demonstration — Voltage in Series-Parallel Circuits To demonstrate the division of voltage across series-parallel circuits, the instructor connects several resistors to form a complex circuit having more than one complete path between the battery terminals. As the in- structor traces several possible paths across the circuit and measures the voltage across each resistance, you see that — regardless of the path chosen — the sum of the voltages for any one path always equals the battery voltage. Also, you see that the voltage drop across resistors of equal value differs, depending on whether they are in a series or parallel part of the circuit and on the total resistance of the path in which they are located. SEEING HOW VOLTAGE DIVIDES IN A SERIES -PARALLEL CIRCUIT VOLTAGE B VOLTAGES A+D=T0TAL VOLTAGE VOLTAGES A+B+C = TOTAL VOLTAGE 2-101 DIRECT CURRENT SERIES-PARALLEL CIRCUITS Review of Series-Parallel Circuits Complex circuits — series-parallel circuits — can be broken down into series and parallel parts so that you may find resistances, currents and voltages. Now you will review the method of breaking down a complex circuit into its basic series and parallel parts. FREAKING DOWN SIMPLE SERIES PAR A 1 I. LI. CIRCUITS Add the series resistances sistances r— *— R a = r 2 + r 3 p Ri tAe*i — combine parallel resistances _. R 1 xR a 1 " R 1 + R a Combine the parallel resist- ances R5 and R Add the series resistances R2, R3 and R4 R a =R2 + R3 + R4 © Combine the series resistances Ri and Rb ■--■I -i; Rt = Ri + Rb s KIRCHHOFF'S LAWS Why Kirchhoff ' s Laws Are Important The total circuit resistance, current and voltage of a complex circuit are easily obtained by breaking down the circuit, if all values for each part of the circuit are known. However, you may find that certain resistances are not known, or that you are only concerned with one part of a circuit. To make the solution of any part of a complex circuit easy, two general rules are used — one concerning current and the other concerning voltage. These rules are Kirchhoff' s laws — the first law for currents and the second law for voltages. You have been using both of these rules or laws, referring to them as rules for current flow and voltage drops in the various types of circuits. Now you are ready to find out more about Kirchhoff' s laws and how they are used to find unknown quantities in any part of a circuit. While the laws relate only to current and voltage, if they are used to find the current and voltage relating to an unknown resistance, the resistance can then be determined by using Ohm's law. 2-103 KIRCHHOFF'S LAWS Kirchhoff's First Law You have found out about current flow in the three types of circuits— series, parallel and series-parallel. You found that the entire circuit current flows through each resistance of a series circuit. In parallel cir- cuits the current divides to flow through more than one path and comes to- gether again after passing through these paths. Series-parallel circuits provide more than one path in some parts of the circuit and only one path in other parts. Regardless of the circuit connections, you found that the current entering a circuit was exactly the same as that leaving the circuit. This is a direct application of Kirchhoff’s First Law, which states that the current entering a junction is equal to the current leaving the junction. The law applies not only to the circuit as a whole but also to every junction with- in the circuit. Thus at a junction of three resistances, where two currents— Ij and 12- in two of the resistances flow toward the junction and one current— 13- in the third resistance flows away from the junction, I3 must equal Ij + 13. HOW ^ works 2-104 KIRCHHOFF'S LAWS Kirchhoff's First Law (continued) In a complete circuit, the current through each resistance will flow to- ward a junction at one end .of the resistance but away from the junction at the other end of the resistance. To use Kirchhoff's first law you should first indicate the current paths through each resistance of the circuit. Then determine which currents flow toward and which flow away from each junction in the circuit. If certain currents are not known, their value and direction both may be determined by applying Kirchhoff's first law. The direction of the unknown current is first determined by comparison of the known currents flowing toward and away from the junction. By adding all the known currents flowing toward the junction and those flow- ing away from the junction, you can determine the direction of the un- known current. At a junction where two currents— Ip and Io— enter the junction and two currents— 13 and I4— leave the junction, if Ip is unknown it may be found by subtracting I 2 from the sum of I3 and I4. Suppose that 12 is 4 amperes, I3 is 6 amperes and I4 is 3 amperes, then Ii is equal to 9 amp (I3 + I4) minus 4 amp (I 2 ) or 5 amp. 1 II + 4A = 6A + 3A II + 4A = 9A 2-105 II = 9A - 4A 1 1 = 5 Amperes KIRCHHOFF'S LAWS Kirchhoff's First Law (continued) This is how you use Kirchhoff's first law to find the unknown currents in a circuit — Suppose your circuit consists of seven resistors Rj, R2> ^3» ^4> ^5) ^6 and R7— connected as shown. If the currents through Rj, R4, Rg and R7 are not known, but the currents and their direction through R 2 , R 3 an< ^ ^5 are known, the unknown currents may be found by applying Kirchhoff's first law to the circuit. THIS IS HOW YOUR CIRCUIT LOOKS R3 *6 In this circuit the current I2 is 7 amperes flowing toward R5, the current Io is 3 amperes flowing toward Rg, and the current I5 is 5 amperes flow- ing toward R7. Draw the circuit in symbol form designating all currents, with values and direction if known. Identify each junction of two or more resistances with a letter. THE CIRCUIT IN SYMBOL FORM 2-106 KIRCHHOFF'S LAWS Kirchhoff's First Law (continued) I* ®’" the unknown currents at all junctions where only one current is unknown, later using these new values to find unknown values at other junctions. From the circuit you can see that junctions A and C have only one unknown. Suppose you start by finding the unknown current at junction A— Of the three currents at junction A— Ii, I2 and I3— both I2 and I3 are known and flow away from the junction. Then Ij must flow to- ward the junction, and its value must equal the sum of I2 and I3 . FINDING I x 'A ”I 2 + l3 . = 7 amp + 3 amp en Ij= 10 amp Next find the unknown current at junction C— At Ctwo currents— 12 and I5— are known and only I4 is unknown. Since I2 flowing toward £ is great- er than I5 flowing away from C, the third current I4 must flow away from £. Also, if the cur- rent flowing toward C equals that flowing away, I2 equals I4 plus I5. FINDING I 4 2-107 12 = 14 + 15 7 amp = 14 + 5 amp Then.14 = 2 amp KIRCHHOFF'S LAWS Kirchhoff's First Law (continued) Now that the value and direction of I4 are known, only Ig is unknown for junction _B. You can find the amount and direction of 16 by applying the law for current at B. FINDING I g I3 and I4 both flow toward B; thus the remaining current 16 must flow away from B. Also 16 must equal the sum of I3 and I4. With 16 known, only I7 remains unknown at junction D. FINDING 1 7 . As I5 and 16 both flow toward junc • tion D, the current I7 must flow away from D and is equal to the sum of I5 an3 16. Re WWW 16 = 5 A P r 7 ( AAAAAAA, 9 iWWW 17 = ? R 5 — WWWV 17 = 15 + 16 1= = 5A You now know all of the circuit currents and their directions through the various resistances. CIRCUIT WITH ALL THE l CURRENTS KNOWN A/VWwJ^> II = 10A R3 mm I 3 = 3A 14 Re mm- 16 = 5A R2 - VWWW- 12 = ?A R5 -mm- I 5 =5A R? mm — 17 - 10A 2-108 KIRCHHOFF'S LAWS Kirchhoff's Second Law While working with the various types of circuits, you found that for any path between the terminals of a voltage source, the sum of the voltage drops across the resistances in each path equaled the voltage of the source. This is one way of using Kirchhoff's second law, which states that the sum of the voltage drops around a circuit equals the voltage applied across the circuit. In the circuit shown, the voltage drops across each resistance differ, but the sum of those in any one path across the terminals add to equal the battery voltage. 1st Circuit Path 2nd Circuit Path - 90 -V Source 3 Voltage 90 V = Total 90 V = Total If more than one voltage source is included in the circuit, the actual volt- age applied to the circuit is the combined voltage of all voltage sources and the voltage drops will be equal to this combined voltage. The com- bined voltage will depend on whether the voltages combine to add or sub- tract. For example, if two batteries are used in the same circuit, they may be connected to either aid or oppose each other. In either case, the total voltage drops across the circuit resistances will equal the sum or difference of the batteries. wm 90V J \i\»r?i 90V f* cutcy 11 1T HTW° i + 40V '45V! \ 90 - 45 v ni,TAGBS ***%£ £& \ ] V.-5V " J U20V J ■\f>V J Battery Voltage = 135V Total Drops = 135V Battery Voltage = 45V Total Drops = 45V 2-109 KIRCHHOFF'S LAWS Kirchhoff's Second Law (continued) Whenever all but one of the voltage drops are known in a path between two junctions, the unknown voltage can be determined by applying Kirchhoff's second law if the voltage between the junctions is known. The junctions may be the terminals of a voltage source or they may be two junctions within the circuit itself. If three resistors— Ri, R2 and R3— are connected in series across a known voltage of 45 volts, and the voltage drops of Ri and R3 are 6 and 19 volts respectively, the voltage drop across R2 is found by applying the law for circuit voltages, Kirchhoff's second law. Finding E2 Ej + E2 + E3 = Et 6V + E2 + 19V = 45V E2 + 25V * 45V Then E2 = 20V Unknown voltages within a complex circuit are found by first finding the voltage across each branch of the circuit, and then, by applying the law, finding the voltage drops across each resistance in the various branches. For series-parallel circuits the voltage across parallel parts of the cir- cuit is used as the total voltage across the various resistances within that part of the circuit. To find the unknown voltages in the series-parallel circuit shown, the law for voltages is applied to each path across the current independently. FINDING TWO UNKNOWN VOLTAGES E t = 90V Finding E2 El + E2 = Et 35 V + E 2 = 90V Then E2 = 55V Finding E3 E3 + E4 = E2 E3 + 20V = 55V Then E3 = 35V 2-110 KIRCHHOFF’S LAWS Demonstration — Kirchhoff's First Law To demonstrate the law of circuit currents, the instructor connects a 15- ohm resistor in series with a parallel combination of three 15 -ohm resis tors and then connects the entire circuit across a 9-volt dry cell battery with a switch and fuse in series. This circuit is shown in the illustration. 2-111 KIRCHHOFF'S LAWS Demonstration — Kirchhoii s First Law (continued) The total resistance of the circuit is 20 ohms, resulting in a total circuit current of 0.45 ampere by Ohm's law. This total current must flow through the circuit from the (-) to (+) battery terminals. At junction (a) the circuit current— 0.45 ampere— divides to flow through the parallel re- sistors toward junction (b). Since the parallel resistors are equal, the current divides equally, with 0.15 ampere flowing through each resistor. At junction (b), the three parallel currents combine to flow away from the junction through the series resistor. As the instructor connects the ammeter to read the current in each lead at the junction, you see that the sum of the three currents flowing toward the junction equals the current flowing away from the junction. CHECK THE CURRENT FLOW AT A CIRCUIT JUNCTION I 3 = 0.15A Current flow ingl [C ur r ent flowing toward the junction)“[away from the junction II + 12 + I 3 = 14 0.15 + 0.15 + 0.15 = 0.45A 2-112 KIRCHHOFF'S LAWS Demonstration— Kirchhoff's Second Law Using the same circuit, the instructor measures the voltage across each resistance in the circuit and also the battery voltage. For each path be- tween the terminals of the battery, you see that the sum of the voltage drops equals the battery voltage. CHECKING THE VOLTAGE DROPS IN EACH PATH THROUGH A CIRCUIT Circuit 1 J Source Voltage Drops) [Voltage (Battery) Next, the instructor connects the resistors in a more complex circuit. Again the voltages of the individual resistors are measured, and you see that the sum of the voltages in any complete path across the circuit equals the battery voltage. 2-113 KIRCHHOFF'S LAWS Review of Kirchhoff's Laws When working with complex circuits, you need to be able to simplify them by redrawing the circuit, combining resistances, using Ohm's law and ap- plying Kirchhoff's laws: Most unknown values in a complex circuit can be found by applying Kirchhoff's laws to either part or all of the circuit. Now let's review these basic laws of circuit currents and voltages. Kirchhoff's First Law The total current entering (flowing toward) a circuit junction equals the total current leaving (flowing away from) the junction. Kirchhoff's Second Law The total voltage drops across the resistances of a closed circuit equal the total voltage applied to the circuit. 1st Circuit Path 2nd Circuit Path +90 2-114 DIRECT CURRENT CIRCUITS Review of Direct Current Circuits Now, as a review, suppose you compare the types of circuits you have found out about and seen in operation. Also review the basic formulas which apply to direct current circuits. SIMPLE CIRCUIT — A single resist- ance connected across a voltage source. WVWW I i — WWM — — WMM — i SERIES CIRCUIT — Resistances con- nected end to end across a voltage source. PARALLEL CIRCUIT — Resistances connected side by side across a com- mon voltage source. SERIES- PARALLEL CIRCUIT — Re- sistances connected partly in series and partly in parallel. 2-115 DIRECT CURRENT CIRCUITS Review of Direct Current Circuits (continued) OHM'S LAW — The current flowing in a circuit changes in the same di- rection that voltage changes, and the opposite direction that resistance changes. Resistance = Voltage Current i=l OHM'S LAW VARIATIONS — Current = Voltage Resistance Voltage = Current x Resistance Resistance = Voltage Current ELECTRIC POWER -- The rate of doing work in moving electrons through a conductor. KIRCHHOFF'S FIRST LAW -- The total current entering (flowing to- ward) a circuit junction equals the total current leaving (flowing away from) the junction. 1 si Circuit P.ith 2nd Circuit Path KIRCHHOFF'S SECOND LAW — The total voltage drops across the re- sistances of a closed circuit equal the total voltage applied to the circuit. 2-116 INDEX INDEX TO VOL. 2 (Note: A cumulative index covering all five volumes in this series will be found at the end of Volume 5.) Ammeters, extending range of, 2-87, 2-88 Demonstration, Current in Series-Parallel Circuits, 2-100 Kirchhoff's First Law, 2-1 1 1, 2-1 12 Kirchhoff's Second Law, 2-1 13 Ohm's Law, 2-34, to 2-36 Ohm's Law and Parallel Resistances, 2-81 to 2-83 Open Circuits, 2-17 to 2-19 Parallel Circuit Current, 2-68 Parallel Circuit Resistance, 2-69 Parallel Circuit Voltages, 2-67 Parallel Resistance, 2-70, 2-71 Power in Parallel Circuits, 2-84 to 2-86 Power in Series Circuits, 2-49 to 2-51 Series Circuit Current, 2-15 Series Circuit Resistance, 2-12 to 2-14 Series Circuit Voltage, 2-16 Series-Parallel Connections, 2-99 Short Circuits, 2-20 to 2-22 Use of Fuses, 2-52, 2-53 Voltage in Series-Parallel Circuits, 2-101 Electric circuits, 2-1, 2-2 Fuses, 2-47, 2-48 Kirchhoff's laws, 2-103 to 2-1 10 Ohm's law, 2-24 to 2-33 parallel circuits, 2-73 Parallel circuit connections, 2-55 Parallel circuits, current flow, 2-57 to 2-59 finding currents, 2-77 to 2-79 power in, 2-80 voltage in, 2-56 Power, electric, 2-42 units, 2-43, 2-44 Power rating of equipment, 2-45, 2-46 Resistance in series, 2-8 to 2-10 Resistances, in parallel, 2-60 to 2-66 in parallel circuits, 2-74 to 2-76 in series-parallel, 2-91 to 2-96 Review, Direct Current Circuits, 2-1 15, 2-1 16 Electric Power, 2-54 Kirchhoff's Laws, 2-114 Ohm's Law, 2-37 Ohm's Law and Parallel Circuits, 2-89 Parallel Circuits, 2-72 Series Circuit Connections, 2-23 Series-Parallel Circuits, 2-102 Series circuit, 2-7 Series circuits, current flow, 2-10 voltage in, 2-1 1 Series-parallel circuit connection, 2-90 Series-parallel circuits, current in, 2-97 voltage in, 2-98 Switches, 2-4, 2-5 Symbols, circuit, 2-6 Voltmeters, extending range of, 2-38 to 2-41 2-119 HOW THIS OUTSTANDING COURSE WAS DEVELOPED: In the Spring of 1951, the Chief of Naval Personnel, seeking a streamlined, more efficient method of presenting Basic Electricity and Basic Electronics to the thousands of students in Navy specialty schools, called on the graphio- logical engineering firm of Van Valkenburgh, Nooger & Neville, Inc., to prepare such a course. This organization, specialists in the production of complete “packaged training programs,” had broad experience serving in- dustrial organizations requiring mass-training techniques. These were the aims of the proposed project, which came to be known as the Common-Core program: to make Basic Electricity and Basic Electronics completely understandable to every Navy student, regardless of previous education; to enable the Navy to turn out trained technicians at a faster rate (cutting the cost of training as well as the time required), without sacrificing subject matter. The firm met with electronics experts, educators, officers-in -charge of various Navy schools and, with the Chief of Naval Personnel, created a dynamic new training course . . . completely up-to-date . . . with heavy emphasis on the visual approach. First established in selected Navy schools in April, 1953, the training course comprising Basic Electricity and Basic Electronics was such a tremendous success that it is now the backbone of the Navy’s current electricity and electronics training program!* The course presents one fundamental topic at a time, taken up in the order of need, rendered absolutely understandable, and hammered home by the use of clear, cartoon-type illustrations. These illustrations are the most effec- tive ever presented. Every page has at least one such illustration — every page covers one complete idea! An imaginary instructor stands figuratively at the reader’s elbow, doing demonstrations that make it easier to understand each subject presented in the course. Now, for the first time, Basic Electricity and Basic Electronics have been released by the Navy for civilian use. While the course was originally de- signed for the Navy, the concepts are so broad, the presentation so clear — without reference to specific Navy equipment — that it is ideal for use by schools, industrial training programs, or home study. There is no finer training material! *“ Basic Electronics.” the second portion of this course, is available as a separate series of volumes. JOHN F. RIDER PUBLISHER, INC, 116 WEST 14th ST., N. Y. 11, N. Y. No. 169-3 basic electricity by VAN VALKENBURGH, NOOGER 6l NEVILLE, INC. VOL. 3 ALTERNATING CURRENT RESISTANCE, INDUCTANCE, CAPACITANCE IN AC REACTANCE AC METERS a RIDER publication $ 2.25 basic electricity by VAN VALKENBIIRGH, NOOGER 6l NEVILLE, INC. VOL. 3 JOHN F. RIDER PUBLISHER, INC. 116 W. 1 4th Street e New York 11, N. Y. First Edition Copyright 1954 by VAN VALKENBURGH, NOOGER AND NEVILLE, INC. All Rights Reserved under International and Pan American Conventions. This book or parts thereof may not be reproduced in any form or in any language without permission of the copyright owner. Library of Congress Catalog Card No. 54-12946 Printed in the United States of America PREFACE The texts of the entire Basic Electricity and Basic Electronics courses, as currently taught at Navy specialty schools, have now been released by the Navy for civilian use. This educational program has been an unqualified success. Since April, 1953, when it was first installed, over 25,000 Navy trainees have benefited by this instruc- tion and the results have been outstanding. The unique simplification of an ordinarily complex subject, the exceptional clarity of illustrations and text, and the plan of pre- senting one basic concept at a time, without involving complicated mathematics, all combine in making this course a better and quicker way to teach and learn basic electricity and electronics. The Basic Electronics portion of this course will be available as a separate series of volumes. In releasing this material to the general public, the Navy hopes to provide the means for creating a nation-wide pool of pre-trained technicians, upon whom the Armed Forces could call in time of national emergency, without the need for precious weeks and months of schooling. Perhaps of greater importance is the Navy’s hope that through the release of this course, a direct contribution will be made toward increasing the technical knowledge of men and women throughout the country, as a step in making and keeping America strong. Van Valkenburgh, Nooger and Neville , Inc. New York, N. Y. October, 1954 iii TABLE OF CONTENTS Vol. 3 — Basic Electricity What Alternating Current Is 3-1 AC Meters 3-22 Resistance in AC Circuits 3-31 Inductance in AC Circuits 3-43 Power in Inductive Circuits 3-72 Capacitance in AC Circuits 3-77 Capacitors and Capacitive Reactance 3-93 V WHAT ALTERNATING CURRENT IS Alternating Current Power Transmission As you probably know, most electric power lines carry alternating current Very little direct current is used for electric lighting and power. There are many good reasons for this choice of AC over DC for electric power transmission. Alternating current voltage can be increased or de- creased easily and without appreciable power loss, through the use of a transformer, while direct current voltages cannot be changed without a considerable power loss. This is a very important factor in the trans- mission of electric power, since large amounts of power must be trans- mitted at very high voltages. At the power station the voltage is "stepped up" by transformers to very high voltages and sent over the transmission line; then at the other end erf the line, other transformers "step down" the voltage to values which can be used for lighting and power. Various kinds of electrical equipment require different voltages for proper operation, and these voltages can easily be obtained by using a trans- former and an AC power line. To obtain such voltages from a DC power lin^requires both a complicated and inefficient circuit. WHAT ALTERNATING CURRENT IS Alternating Current Power Transmission (continued) Since the power transmitted equals the voltage multiplied by the current (P = El), and the size of the wire limits the maximum current which can be used, the voltage must be increased if more power is to be transmitted over the same size wires. Also, excessive current flow causes over- heating of the wires, resulting in large power loss, so that the maximum current is kept as low as possible. The voltage, however, is limited only by the insulation of the transmission line. Since the insulation can be easily strengthened, the voltage can be increased considerably, permitting the transfer of large amounts of power with smaller wires and much less power load. IN POWER TRANSMISSION THE CURRENT IS LIMITED BY When current flows through a wire to reach the electrical device using pow- er, there is a power loss in the wire proportional to the square of the cur- rent (P =i2r). Any reduction in the amount of current flow required to transmit power results in a reduction in the amount of power lost in the transmission line. By using high voltage, lower current is required to transmit a given amount of power. Transformers are needed to raise the voltage for power transmission and lower it for use on electric power lines and, since transformers can only be used with AC, nearly all elec- tric power lines are AC rather than DC. POWER LOSS IN TRANSMISSION IS PROPORTIONAL TO THE SQUARE OF THE CURRENT Power loss 3-2 WHAT ALTERNATING CURRENT IS DC and AC Current Flow Alternating current — AC — flows back and forth in a wire at regular inter- vals, going first in one direction and then the other. You know that direct current flows only in one direction, and that current is measured by count- ing the number of electrons flowing past a point in a circuit in one second. Suppose a coulomb of electrons moves past a point in a wire in one second with all of the electrons moving in the same direction; the current flow then is one ampere DC. If a half coulomb of electrons moves in one di- rection past a point in a half second, then reverses direction and moves past the same point in the opposite direction during the next half second, a total of one coulomb of electrons passes the point in one second — and the current flow is one ampere AC. COMPARING IN A WIRE 1st Half Second Reference point one arrow 1/2 coulomb mm 2nd Half Second C WHAT ALTERNATING CURRENT IS Waveforms Waveforms are graphical pictures showing how voltages and currents vary over a period of time. The waveforms for direct current are straight lines, since neither the voltage nor current vary for a given cir- cuit. If you connect a resistor across a battery and take measurements of voltage across, and current through, the resistor at regular intervals of time, you find no change in their values. Plotting the values of E and I, each against time, you obtain straight lines — the waveforms of the circuit voltage and current. WAVEFORMS ARE PICTURES OF VOLTAGE OR CURRENT VARIATIONS SECONDS DC WAVEFORMS Imagine that you have a zero-center voltmeter and ammeter, and can take readings above and below zero when the polarity of the measured voltage and current is reversed. H you reverse the battery leads while taking the measurements, you find that the waveforms consist of two straight lines — one above and one below zero. By connecting the ends of these lines to form a continuous line, you can obtain the waveforms of voltage and cur- rent. These waveforms show that the current and voltage are AC rather than DC, since they indicate the changing direction of the current flow and the reversal in polarity of the voltage. AC WAVEFORMS 3-4 WHAT ALTERNATING CURRENT IS Waveforms (continued) Another type of waveform is pulsating direct current, which represents variations in voltage and current flow without a change in the direction of current flow. This waveform is common to the DC generator, since the generator output contains a ripple or variation due to commutator action. Battery waveforms do not vary unless the circuit itself changes, such as reversing the battery terminals to obtain an AC waveform. If , in a circuit consisting of a resistor and a switch connected across a battery, you open and close the switch, causing the current to stop and start but not to reverse direction, the circuit current is pulsating direct current. The waveforms for this pulsating current resemble AC waveforms, but do not go below zero since the current does not change its direction. PULSATING DC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Seconds Waveforms of voltage and current are not always made of straight lines connecting points. In most cases waveforms are curved, representing gradual changes in voltage and current. This is particularly true of pul- sating DC waveforms. Also, pulsating direct current does not always vary between zero and a maximum value, but may vary over any range between these values. The waveform of a DC generator is pulsating DC and does not fall to zero but, instead, varies only slightly below the maximum value. OTHER WAVEFORMS OF PULSATING DC 2 WHAT ALTERNATING CURRENT IS Waveforms (continued) The waveforms of mostalternating currents are curved to represent grad - ual changes in voltage and current, first increasing then decreasing in value for each direction of current flow. Most of the alternating current which you will use has a waveform represented by a sine curve, which you will find out about a little later. While alternating currents and voltages do not always have waveforms which are exact sine curves, they are nor- mally assumed to have a sine waveform unless otherwise stated. When direct current and alternating current voltages are both present in the same circuit, the resulting voltage waveform is a combination of the two voltages. The AC wave is added to the DC wave, with the value of th€ DC voltage becoming the axis from which the AC wave moves in each di rection. Thus the maximum point of DC voltage replaces the zero value as the AC waveform axis. The resulting waveform is neither DC nor AC and is called "super -imposed AC," meaning that the AC wave is added to or placed over the DC wave. When AC and DC are added together, the AC axis shifts, resulting in "Superimposed AC" DC waveform •+■ AC waveform Superimposed AC waveform 3-6 WHAT ALTERNATING CURRENT IS Alternating Current Cycles When the waveform of an AC voltage or current passes through a com- plete set of positive and negative values, it completes a cycle. AC cur- rent first rises to a maximum and falls to zero in one direction, then rises to maximum and falls to zero in the opposite direction. This com- pletes a cycle of AC current and the cycle repeats as long as the current flows. Similarly, AC voltage first rises to a maximum and falls to zero in one polarity, then rises to maximum and falls to zero in the opposite polarity to complete a cycle. Each complete set of both positive and neg- ative values of either voltage or current is a cycle. On the next sheet you will see that an AC generator consists of a coil of wire rotating in a magnetic field between two opposite magnetic poles, and that each time a side of the coil passes from one pole to the other the cur- rent flow generated in the coil reverses its direction. In passing two op- posite poles, the current flows first in one direction and then the other, completing a cycle of current flow. A CYCLE IS A COMPLETE SET OF POSITIVE AND NEGATIVE VALUES 3 - WHAT ALTERNATING CURRENT IS Elementary Generator Construction An elementary generator consists of a loop of wire placed so that it can be rotated in a stationary magnetic field to cause an induced current in the loop. Sliding contacts are used to connect the loop to an external circuit in order to use the induced current. The pole pieces are the north and south poles of the magnet which supplies the magnetic field. The loop of wire which rotates through the field is called the "armature." The ends of the armature loop are connected to rings called "slip rings," which rotate with the armature. Brushes ride up against the slip rings to pick up the electricity generated in the armature and carry it to the external circuit. the ELEMENTARY GENERATOR In the description of the generator action as outlined on the following sheets, visualize the loop rotating through the magnetic field. As the sides of the loop cut through the magnetic field, they generate an induced emf which causes a current to flow through the loop, slip rings, brushes, zero-center current meter and load resistor— all connected in series. The induced emf that is generated in the loop, and therefore the current that flows, depends upon the position of the loop in relation to the magnetic field. Now you are going to analyze the action of the loop as it rotates through the field. 3-8 WHAT ALTERNATING CURRENT IS Elementary Generator Operation Here is the way the elementary generator works. Assume that the arma- ture loop is rotating in a clockwise direction, and that its initial position is at A (zero degrees). In position A, the loop is perpendicular to tl^e mag- netic field and the black and white conductors of the loop are moving par- allel to the magnetic field. If a conductor is moving parallel to a magnetic field, it does not cut through any lines of force and no emfcan be generated in the conductor. This applies to the conductors of the loop at the instant they go through position A — no emf is induced in them and, therefore, no current flows through the circuit. The current meter registers zero. As the loop rotates from position A to position B, the conductors are cutting through more and more lines of force until at 90 degrees (position B) they are cutting through a maximum number of lines of force. In other words, between zero and 90 degrees, the induced emf in the conductors builds up from zero to a maximum value. Observe that from zero to 90 degrees the black conductor cuts down through the field while at the same time the white conductor cuts up through the field. The induced emfs in both conductors are therefore in series -adding, and the resultant voltage across the brushes (the terminal voltage) is the sum of the two induced emfs, or double that of one conductor since the induced voltages are equal to each other. The cur- rent through the circuit will vary just as the induced emf varies — being zero at zero degrees and rising up to a maximum at 90 degrees. The cur- rent meter deflects increasingly to the right between positions A and B, in- dicating that the current through the load is flowing in the direction shown. The direction of current flow and polarity of the induced emf depend on the direction of the magnetic field and the direction of rotation of the armature loop. The waveform shows how the terminal voltage of the elementary gen- erator varies from position A to position B. The simple generator drawing on the right is shown shifted in position to illustrate the relationship be- tween the loop position and the generated waveform. HOW THE ELEMENTARY GENERATOR WORKS 3-9 WHAT ALTERNATING CURRENT IS Elementary Generator Operation (continued) As the loop continues rotating from position B (90 degrees) to position C (180 degrees), the conductors which are cutting through a maximum num- ber of lines of force at position B cut through fewer lines, until at po- sition C they are moving parallel to the magnetic field and no longer cut through any lines of force. The induced emf therefore will decrease from 90 to 180 degrees in the same manner as it increased from zero to 90 degrees. The current flow will similarly follow the voltage variations. The generator action at positions B and C is illustrated. WHAT ALTERNATING CURRENT IS Elementary Generator Operation (continued) From zero to 180 degrees the conductors of the loop have been moving in the same direction through the magnetic field and, therefore, the polarity of the induced emf has remained the same. As the loop starts rotating be- yond 180 degrees back to position A, the direction of the cutting action of the conductors through the magnetic field reverses. Now the black con- ductor cuts up through the field, and the white conductor cuts down through the field. As a result, the polarity of the induced emf and the current flow will reverse. From positions C through D back to position A, the current flow will be in the opposite direction than from positions A through C. The generator terminal voltage will be the same as it was from A to C except for its reversed polarity. The voltage output waveform for the complete revolution of the loop is as shown. 3-11 WHAT ALTERNATING CURRENT IS Elementary Generator Output Suppose you take a closer look at the output waveform of the elementary generator and study it for a moment. How does it compare to the voltages with which you have been dealing up to this time? The only voltages you have used so far are DC voltages like those obtained from a battery. A DC voltage can be represented as a straight line whose distance above the zero reference line depends upon its value. The diagram shows the DC voltage next to the voltage waveform put out by the elementary AC gen- erator, You see the generated waveform does not remain constant in value and direction, as does the DC curve. In fact, the generated curve varies continuously in value and is as much negative as it is positive. The generated voltage is therefore not DC voltage, since a DC voltage is defined as a voltage which maintains the same polarity output at all times. The generated voltage is called an "alternating voltage," since it alternates periodically from plus to minus. It is commonly referred to as an AC voltage — the same type of voltage that you get from the AC wall socket. The current that flows, since it varies as the voltage varies, must also be alternating. The current is also referred to as AC current. AC current is always associated with AC voltage — an AC voltage will always cause an AC current to flow. A B C D A the AC waveform 3-12 WHAT ALTERNATING CURRENT IS Alternating Current Frequency When the armature of an AC generator is rotating, the faster the arma- ture coil turns past the magnetic poles the more often the current re- verses each second. Therefore it completes more cycles per second, since each current reversal ends a half cycle of current flow. The num- ber of cycles per second is "frequency." Alternating current frequency is important to understand, since most AC electrical equipment requires a specific frequency as well as a specific voltage and current for proper operation. The standard commercial fre- quency used in this country is 60 cycles per second. Lower frequencies cause flicker when used for lighting, since each time the current changes direction it falls to zero — turning an electric lamp off momentarily. With 60 cycles, the lamp turns on and off 120 times each second; however, no flicker is noticeable since the eye cannot react fast enough to see the light turn off. FREQUENCY IS THE NUMBER OF CYCLES PER SECOND If 15 cycles are completed in 1/4 second, the frequency is 60 cycles per second. 3-13 WHAT ALTERNATING CURRENT IS May i mum and Peak-to-Peak Values of a Sine Wave Suppose you compare a half cycle of an AC sine wave to a DC waveform for the same length of time. If the DC starts and stops at the same mo- ment as the half-cycle sine wave and each rises to the same maximum value, the DC values are greater than the corresponding AC values at all points except the pointat which the AC sine wave passes through its maxi- mum value. At this point the DC and AC values are equal. This point on the sine wave is the maximum or peak value. COMPARISON of DC and AC WAVEFORMS Maximum Value of DC Waveform For each complete cycle of AC there are two maximum or peak values, one for the positive half cycle and the other for the negative half cycle. The difference between the peak positive value and the peak negative value is called the peak-to-peak value of a sine wave. This value is twice the maximum or peak value of the sine wave and is sometimes used for measurement of AC voltages. An oscilloscope and certain types of AC voltmeters measure peak-to-peak values of AC voltages at the input and output of radio amplifiers, phonograph amplifiers, etc.; but usually AC voltages and currents are expressed in effective values (a term you will find out about later), rather than peak-to-peak values. Maximum Positive Value 3-14 WHAT ALTERNATING CURRENT IS Average Value of a Sine Wave In comparing a half-cycle AC sine wave to a DC waveform you found that the AC instantaneous values are all less than the DC value except at the peak value of the sine wave. Since all points of the DC waveform are equal to the maximum value, this value is also the average value of the DC wave. The average value of a half cycle of the AC sine wave is less than the peak value, since all but one point on the waveform are lower in value. For all sine waves, the average value of a half cycle is 0.637 of the maximum or peak value. This value is obtained by averaging all the instantaneous values of the sine wave (for a half cycle). Since the shape of the sine wave does not change, even though its maximum value changes, the average value of any sine wave is always 0.637 of the peak value. waveforms DC Waveform Average value DC average value equals maximum value AC Waveform While an AC sine wave with a maximum value of 1 ampere has an aver- age value of 0.637 ampere for each half cycle, the power effect of a 1- ampere AC current is not the same as that of a 0.637-ampere DC current. For this reason, average values of AC current and voltage waves are not often used . 3-15 WHAT ALTERNATING CURRENT IS Effective Value of a Sine Wave If a direct current flows through a resistance, the resulting energy con- verted into heat equals I 2 R, or E 2 /R in watts. An alternating current with a maximum value of 1 ampere, for instance, is not expected to produce as much heat as a direct current of 1 ampere, as alternating current does not maintain a constant value. The rate at which heat is produced in a resistance forms a convenient basis for establishing an effective value of alternating current, and is known as the "heating effect" method. An alternating current is said to have an effective value of one ampere when it will produce heat in a given resistance at the same rate as does one ampere of direct current. HEATING EFFECT OF ONE AMPERE OF DC AND AC DC AND AC MAXIMUM WAVEFORM The Heating Effect Of 1 Maximum AC Ampere EFF _ xhe Heating Effect Of 1 Maximum DC Ampere AC t 707° F A EFF - 1000° F AC IeFF ■ • Wf 3-16 WHAT ALTERNATING CURRENT IS Effective Value of a Sine Wave (continued) The effective value of a sine wave of current may be computed to a fair de- gree of accuracy by taking equally spaced instantaneous values and ex- tracting the square root of their average, squared values. For this reason, the effective value is often called the "root-mean-square" (rms) value. By this method or by means of higher mathematics it may be shown that the effective value (I) of any sine-wave current is always 0. 707 times the maximum value (Imax). Since alternating currents are caused to flow by alternating emf's, the ratio between effective and maximum values of emf's is the same as for currents. The effective, or rms, value (E) of a sine-wave emf is 0. 707 times the maximum value (Emax). When an alternating current or voltage is specified, it is always the effec- tive value that is meant unless there is a definite statement to the contrary. It should be noted that all meters, unless marked to the contrary, read ef- fective values of current and voltage. 3-17 WHAT ALTERNATING CURRENT IS Transformers Electrical energy requires a convenient means for conversion, and for transfer from circuit to circuit. A device called a transformer is ideally suited for these purposes. Transformers change voltages from one level to another as needed, and transfer energy from one circuit to another with great efficiency. Transformers are generally composed of two coils placed close to each other but not connected together. The coil which receives energy from the line voltage source, etc. , is called the "primary" and the coil which delivers energy to a load is called the "secondary. " Even though the coils are not physically connected together they manage to convert and transfer energy as required. This action is complex and is explained in detail later. Some transformers take a high input voltage at the primary and deliver a low output voltage at the secondary. These are called "step-down" trans- formers. On the other. hand, "step-up" transformers take a low input voltage at the primary and deliver a high output voltage at the secondary. The transformer used in the demonstration on the following sheets is of the step-down variety because it takes 117-volts, AC, at the primary and delivers 6.3 -volts, AC, from the secondary as shown below. One variation in the transformer family is found in a device called the autotransformer. This device uses only one coil to do all the work. There are two input ("primary") leads and at least two output ("secondary") leads. The primary and secondary leads are attached to, or tapped off, the same coil; the extent to which the input voltage is changed is determined by the points at which the secondary leads are attached. These devices deliver step-up and step-down voltage— just as transformers do. One type of variable transformer, called a powerstat, is sometimes used as a voltage source in laboratories. The powerstat permits the instructor to make voltage settings as desired In some schools you may not use the transformer shown below at all but will use a powerstat instead for all demonstrations described in this sec- tion. MEASURING THE TRANSFORMER'S SECONDARY VOLTAGE Primary Secondary JL l Transformer Symbol 3-18 WHAT ALTERNATING CURRENT IS Demonstration — AC Voltmeter Although calibrated to read the effective value of AC voltages, AC volt- meters can also be used to measure the approximate value of a DC voltage. To show how the effective value of an AC voltage compares to a DC volt- age, the instructor uses an AC voltmeter to measure both the DC voltage of a 7.5-volt battery and the effective AC voltage output of a 6.3- volt transformer. Five dry cells are connected to form a 7. 5 -volt battery, and the 0-25 volt AC voltmeter is used to measure the voltage across the battery terminals. You see that the meter reading is approximately 7.5 volts, but the reading is not as accurate as it would be if a DC voltmeter were used. MEASURING A BATTERY VOLTAGE WITH AN AC VOLTMETER Next, the instructor connects the 117- volt primary lead of the trans- former across the AC power line. The voltage across the secondary leads is then measured with the AC voltmeter, and you see that it is approxi- mately 7.5 volts. Although the transformer is rated at 6.3 volts AC, the secondary voltage will always be higher than its rated value when the transformer is not furnishing power. The size of the load determines the exact value of the secondary voltage. In comparing the measured voltages —7.5 volts DC and 7.5 volts AC— you find that the two meter readings are nearly the same. Some difference in the readings should be expected, as the AC voltage is approximately 7.5 volts effective while the DC voltage is exactly 7.5 volts. MEASURING THE TRANSFORMER'S SECONDARY VOLTAGE 3-19 WHAT ALTERNATING CURRENT IS Demonstration — Effective Value of AC Voltage To show that the 7.5 volts effective AC has the same effect as 7.5 volts DC, the 7.5-volt battery and the 6.3-volt transformer are each used to light the same type of lamp. Although the transformer is furnishing power, the load is light enough so that for all practical purposes the effective AC voltage can be assumed to be 7.5 volts. One lamp socket is connected across the battery and another is connected across the secondary leads of the transformer. The instructor then in- serts identical lamps in each socket, and you see that the brightness of the two lamps is the same. This shows that the power effect of the two volt- ages is the same. 3-20 WHAT ALTERNATING CURRENT IS Review of Alternating Current Alternating current differs from direct current not only in its waveform and electron movement but also in the way it reacts in electrical circuits. Before finding out how it reacts in circuits you should review what you have already found out about AC and the sine wave. ALTERNATING CURRENT — Current flow which is constantly changing in amplitude, and re- verses its direction at regular intervals. 1st Hall Second • 44044 * 2nd Half Second WAVEFORM — A graphical pic- ture of voltage or current varia- tions over a period of time. DC WAVEFORM SINE WAVE — A continuous curve of all the instantaneous values of an AC current or voltage. CYCLE — A complete set of pos- itive and negative values of an AC current or voltage wave. FREQUENCY — The number of cycles per second. MAXIMUM, EFFECTIVE, AND AVERAGE VALUES of a sine~¥i7e. 3-21 AC METERS Why DC Meters Cannot Measure AC There are noticeable differences, particularly in the scales, between DC and AC voltmeters. There is also a basic difference in the meter move- ments themselves. DC meters use a basic moving-coil meter movement in which the moving coil is suspended in the magnetic field between the poles of a permanent magnet. Current flow through the coil in the correct direction (polarity) causes the coil to turn, moving the meter pointer up-scale. However, you will recall that a reversal of polarity causes the moving coil to turn in the opposite direction, moving the meter pointer down below zero. EFFECT OF CURRENT REVERSAL IN A J) Q METER If an AC current were passed through a basic DC meter movement, the moving coil would turn in one direction for a half cycle, then — as the cur- rent reversed direction— the moving coil would turn in the opposite direc- tion. For ordinary 60 cycles the pointer would be unable to follow the re- versal in current fast enough, and the pointer would vibrate back and forth at zero, the average position of the AC wave. The greater the current flow the further the pointer would attempt to swing back and forth and, in a short time, the excess vibration would break the needle. Even if the pointer could move back and forth fast enough, the speed of movement would be so great that you could not obtain a meter reading. HOW AC CURRENT AFFECTS A DC METER 3-22 AC METERS Rectifier Type AC Voltmeters A basic DC meter movement may be used to measure AC through the use of a rectifier— a device which changes AC to DC. The rectifier permits current flow in only one direction so that, when AC tries to flow through it, current only flows for a half of each complete cycle. The effect of such a rectifier on AC current flow is illustrated below. RECTIFIERS CHANGE AC TO DC f\J\j r\y\j Normal AC current flow in a wire Direction of current flow Rectifier symbol A_A_ Pulsating DC Rectifier allows current flow in one direction only If the rectifier is connected in series with a basic DC meter movement so that it permits current flow only in the direction necessary for correct meter polarity, the meter current flows in pulses. Since these pulses of current are all in the same direction, each causes an up-scale deflection of the meter pointer. The pointer cannot move rapidly enough to return to zero between pulses, so that it continuously indicates the average value of the current pulses. The meter reads the average of DC pulses a/VmETER WITH RECTIFIER MEASURES AC CURRENT Rectifier 3-23 AC METERS Rectifier Type AC Voltmeters (continued) When certain metallic materials are pressed together to form a junction, the combination acts as a rectifier having a low resistance to current flow in one direction and a very high resistance to current flow in the opposite direction. This action is due to the chemical properties of the combined materials. The combinations usually used as rectifiers are copper and copper oxide, or iron and selenium. Dry metal rectifiers are constructed of disks ranging in size from less than a half inch to more than six inches in diameter. Copper-oxide rectifiers consist of disks of copper coated on one side with a layer of copper oxide while selenium rectifiers are con- structed of iron disks coated on one side with selenium. DRY METAL RECTIFIERS Copper — Copper Oxide Coating COPPER OXIDE RECTIFIER Selenium Coating SELENIUM RECTIFIER Dry metal rectifier elements (an element is a single disk) are generally made in the form of washers which are assembled on a mounting bolt.in any desired series or parallel combination, to form a rectifier unit. The symbol shown below is used to represent a dry metal rectifier of any type. Since these rectifiers were made before the electron theory was used to determine the direction of current flow, the arrow points in the direction of conventional current flow but in the direction opposite to the electron flow. Thus the arrow points in opposite direction to that of the current flow as used in electronics. DRY METAL RECTIFIER SYMBOL ELECTRON current flow opposite direction from symbol arrow 3-24 AC METERS Rectifier Type AC Voltmeters (continued) Each dry metal rectifier element will stand only a few volts across its terminals but by stacking several elements in series the voltage rating is increased. Similarly each element can pass only a limited amount of cur- rent. When greater current is desired several series stacks are con- nected in parallel to provide the desired amount of current. tenieA, btadcin# atcneaMA, t&e VOLTAGE RATING a cOuf metal nectifyen. ~ fiana/tU cohmccUok totouoMi, me CURRENT RATING. Dry metal rectifiers are very rugged and have an almost unlimited life if not abused. Because of the low voltage rating of individual units they are normally used for low voltages (130 volts or less) since it becomes im- practical to connect too many elements in series. By paralleling stacks or increasing the diameter of the disks, the current rating can be in- creased to several amperes so that they are often used for low-voltage- high-current applications. Very small units are used to measure AC volt- age on a DC voltmeter. Larger units are used in battery chargers and various types of power supplies for electronic equipment. 3-25 AC METERS Rectifier Type AC Voltmeters (continued) Rectifier type AC meters are only used as voltmeters and the meter range is determined and changed in the same manner as that of a DC voltmeter . They cannot be used for current measurement, since ammeters are con- nected in series with the line current and a rectifier type meter so con- nected would change the AC circuit current to DC, which is not desirable. Various AC rectifier type meter circuits are illustrated below: SIMPLE METER RECTIFIER CIRCUIT .... 1. A simple meter rectifier circuit consists of a multiplier, rectifier and basic meter movement con- nected in series. For one half cycle, current flows through the meter circuit. During the next half cycle, no current flows, al- though a voltage exists across the circuit including the rectifier. ADDING A RECTIFIER TO THE SIMPLE METER CIRCUIT 2. To provide a return path for the AC current half-cycle pulses not used to operate the meter move- ment, an additional rectifier is connected across the meter recti- fier and meter movement. The unused pulses flow through this branch— not through the meter. BRIDGE RECTIFIER CIRCUIT .... 3. A bridge circuit using four recti- fiers is sometimes used. It is so connected that both halves of the AC current wave must follow paths that lead through the meter in the same direction. Thus, the number of current pulses flowing through the meter movement is doubled. Because the meter reading is the average of the half-cycle current pulses, the scale is not the same as that used for DC. Although the amount of de- flection is a result of average current flow through the meter movement, the scale is calibrated to read effective values of voltage. 3-26 AC METERS Moving-Vane Meter Movements A meter which you can use to measure both AC current and voltage is the moving-vane meter movement. The moving-vane meter operates on the principle of magnetic repulsion between like poles. The current to be measured flows through a field coil producing a magnetic field proportional to the strength of the current. Suspended in this field are two iron vanes— one fixed in position, the other movable and attached to the meter pointer. The magnetic field magnetizes these iron vanes with the same polarity re- gardless of the direction of current flow in the coil. Since like poles repel, the movable vane pulls away from the fixed vane moving the meter pointer This motion exerts a turning force against a spring. The distance the vane will move against the force of the spring depends on the strength of the magnetic field, which in turn depends on the coil current. THE MOVING -VANE METER MOVEMENT Moving-vane meters may be used for voltmeters, in which case the field coil consists of many turns of fine wire which generate a strong field with only a small current flow. Ammeters of this type use fewer turns of a heavier wire, and depend on the larger current flow to obtain a strong field. These meters are generally calibrated at 60 cycles AC, but may be used at other AC frequencies. By changing the meter scale calibration, moving - vane meters will measure DC current and voltage. 3-27 AC METERS Hot-Wire and Thermocouple Meters Hot-wire and thermocouple meters both utilize the heating effect of cur- rent flowing through a resistance to cause meter deflection, but each uses this effect in a different manner. Since their operation depends only on the heating effect of current flow, they may be used to measure direct current and alternating current of any frequency. The hot-wire ammeter deflection depends on the expansion of a high re- sistance wire caused by the heating effect of the wire itself as current flows through it. A resistance wire is stretched taut between the two me- ter terminals with a thread attached at a right angle to the center of the wire. A spring connected to the opposite end of the thread exerts a con- stant tension on the resistance wire. Current flow heats the wire, causing it to expand. This motion is transferred to the meter pointer through the thread and a pivot. The thermo-couple meter consists of a resistance wire across the meter terminals which heats in proportion to the amount of current flow. Attached to this heating resistor is a small thermo-couple junction of two unlike metal wires which connect across a very sensitive DC meter movement. As the current being measured heats the heating resistor, a small current (which flows only through the thermo-couple wires and the meter movement) is generated by the thermo-couple junction. The current being measured flows only through the resistance wire, not through the meter movement itself. The pointer turns in proportion to the amount of heat generated by 3-28 AC METERS Electrodynamometer Movements An electrodynamometer movement utilizes the same basic operating prin- ciple as the basic moving-coil DC meter movement, except that the per- manent magnet is replaced by fixed coils. A moving coil to which the me- ter pointer is attached is suspended between two field coils and connected in series with these coils. The three coils (two field coils and the moving coil) are connected in series across the meter terminals so that the same current flows through each. Current flow in either direction through the three coils causes a magnetic field to exist between the field coils. The current in the moving coil causes it to act as a magnet and. exert a turning force against a spring. If the cur- rent is reversed, the field polarity and the polarity of the moving coil re- verse simultaneously, and the turning force continues in the original direc- tion. Since reversing the current direction does not reverse the turning force, this type of meter can be used to measure both AC and DC current. While some voltmeters and ammeters use the dynamometer principle of operation, its most important application is in the wattmeter about which you will find out a little later. 3-29 AC METERS Review of AC Meters To review the principles and construction of AC meters, suppose you com- pare the various meter movements and their uses. Although there are other types of meters used for AC, you have found out about those which are most commonly used. RECTIFIER TYPE AC METER — A basic DC meter movement with a rectifier connected to change AC to DC . Commonly used as an AC volt- meter. MOVING-VANE METER — Meter which operates on magnetic repul- sion principle, using one movable and one fixed vane. Can be used on AC or DC to measure either voltage or current. HOT-WIRE AMMETER — Meter movement based on the expansion of a wire heated by current flowthrough it. Used only to measure current. THERMO-COUPLE AMMETER — Meter movement utilizing the heat of a resistor through which current flows to develop a measurable cur- rent in a thermo-couple. E LECTRODYNAMOMETERS — Commonly used in wattmeters rath- er than voltmeters and ammeters. Basic principle is identical to that of a D'Arsonval movement except that field coils are used instead of a permanent magnet. 3-30 RESISTANCE IN AC CIRCUITS AC Circuits Containing Resistance Only Many AC circuits consist of pure resistance, and for such circuits the same rules and laws apply as for DC circuits. Pure resistance circuits are made up of electrical devices which contain no inductance or capaci- tance (you will find out about inductance and capacitance a little later). Devices such as resistors, lamps and heating elements have negligible in- ductance or capacitance and for practical purposes are considered to be made up of pure resistance. When only such devices are used in an AC circuit, Ohm's Law, Kirchhoff's Laws and the circuit rules for voltage, current and power can be used exactly as in DC circuits. In using the circuit laws and rules you must use effective values of AC voltage and current. Unless otherwise stated, all AC voltage and current values are given as effective values. Other values such as peak-to-peak voltages measured on the oscilloscope must be changed to effective values before using them for circuit computations. 3-31 RESISTANCE IN AC CIRCUITS Current and Voltage in Resistive Circuits When an AC voltage is applied across a resistor the voltage increases to a maximum with one polarity, decreases to zero, increases to a maximum with the opposite polarity and again decreases to zero, to complete a cycle of voltage. The current flow follows the voltage exactly: as the voltage increases the current increases; when the voltage decreases the current decreases; and at the moment the voltage changes polarity the current flow reverses its direction. Because of this, the voltage and current waves are said to be "in phase." CURRENT AND VOLTAGE ARE IN PHASE IN RESISTIVE CIRCUITS 1 90° 180 Time KA \J Sine waves of voltages or currents are "in phase" whenever they are of the same frequency and pass through zero simultaneously, both going in the same direction. The amplitude of two voltage waves or two current waves which are "in phase" are not necessarily equal, however. In the case of "in phase" current and voltage waves, they are seldom equal since they are measured in different units. In the circuit shown below the effec- tive voltage is 6.3 volts, resulting in an effective current of 2 amperes, and the voltage and current waves are "in phase." 6-volt, 500-ma. Lamps RESISTANCE IN AC CIRCUITS Power in AC Circuits The power used in an AC circuit is the average of all the instantaneous values of power or heating effect for a complete cycle. To find the power, all of the corresponding instantaneous values of voltage and current are multiplied together to find the instantaneous values of power, which are then plotted for the corresponding time to form a power curve. The aver- age of this power curve is the actual power used in the circuit. For "in phase" voltage and currentwaves, all of the instantaneous powers are above the zero axis and the entire power curve is above the zero axis. This is due to the fact that whenever two positive values are multiplied together the result is positive, and whenever two negative values are multiplied together the result is also positive. Thus, during the first half cycle of E and I, the power curve increases in a positive direction from zero to a maximum and then decreases to zero just as the E and I waves do. During the second half cycle, the power curve again increases in a positive direction from zero to maximum and then decreases to zero while E and I both increase and decrease in the negative direction. Notice that if a new axis is drawn through the power wave, halfway between its maxi- mum and minimum values, the power wave frequency is twice that of the voltage and current waves. When two numbers— each being less than 1— are multiplied together , the result is a smaller number than either of the original numbers— for example, 0.5V x 0.5A = 0.25W. For that reason, some or all of the instan- taneous values of a power wave may be less than those for the circuit cur- rent and voltage waves. 3-33 RESISTANCE IN AC CIRCUITS Power in Resistive Circuits A line drawn through the power wave exactly halfway between its maximum and minimum values is the axis of the power wave. This axis represents the average value of power in a resistive circuit, since the shaded areas above the axis are exactly equal in area to those below the axis. Average power is the actual power used in any AC circuit. Since all the values of power are positive for AC circuits consisting only of resistance, the power wave axis and the average power for such circuits is equal to exactly one-half the maximum, positive, instantaneous power value. This value can also be found by multiplying the effective values of E and I together. This applies to AC circuits containing resistance only, since AC circuits containing inductance or capacitance may have negative instantaneous power values. IN AC CIRCUITS WITH RESISTANCE ONLY Pav = Pmax AVERAGE POWER ^(watts) Shaded areas above axis of - average power / I 1 \ below the axis. j \ i j:::i i ( “t.1 *i H | J \ |(volts) l J uverag: POWER 1 III f IS / \ &/ | (amps)\ 1! / \ wJ s 1 / V f 90° ^ tj 270° \m Since Pmax = Emax x I max \ „ Emax x Imax \ Pav = 2 \ Since Emax = 1.414 Eeff and Imax = 1.414 Ieff 1.414 Eeff x 1.414 Ieff X p av = \ 1.414 x 1.414 x Eeff Ieff Since 1.414 x 1.414 = 2, Pav = Eeff x Ieff or P = El 3-34 RESISTANCE IN AC CIRCUITS Power Factor When Ieff and Eeff are in phase, the product is power in watts the same as in DC circuits. As you will find out later, the product of Ieff and Eeff is not always power in watts, but is called "volt-amperes". The power in watts becomes I 2 R or E z /R or Power used in the resistive part of the circuit. While a source may produce volts and amperes the power in watts may be small or zero. The ratio between the power in watts of a circuit and the volt -amperes of a circuit is called "power factor. " In a pure resistive cir- cuit power in watts is equal to Ieff x Eeff so "power factor" in a pure re- sistive circuit is equal to power in watts divided by volt-amperes which equals 1 (one). Power factor is expressed in percent or as a decimal. POWER FACTOR IN RESISTIVE CIRCUITS I 2 R or E 2 /R = Eeff x Ieff I 2 R or E 2 /R = Watts „ . I 2 R E 2 /R 1000 Power Factor = ^ x * eff or ^nTlif f = l000 = *• 100% Power Factor = rr Volt-Amperes Power Factor = 1. 0 or 100%in a pure resistive circuit 3-35 RESISTANCE IN AC CIRCUITS Wattmeters While power may be computed from the measured effective values of E ‘and I in AC circuits containing only resistance, it can be measured directly with a wattmeter. Wattmeters are not used as commonly as the meters with which you are familiar — voltmeters, ammeters and ohmmeters — but in order to find out about AC circuits you will need to use them. Since wattmeters work differently than the meters you have used and are easily damaged if connected incorrectly, you must find out how to operate them properly. You see the wattmeter looks very much like any other type of meter, ex- cept that the scale is calibrated in watts and it has four terminals, instead of the usual two. Two of these terminals are called the "voltage terminals" and the other two are called the "current terminals." The voltage ter- minals are connected across the circuit exactly as a voltmeter is connected, while the current terminals are connected in series with the circuit cur- rent in the same manner as an ammeter is connected. Two terminals — one voltage terminal and one current terminal — are marked t. In using the wattmeter, these two terminals must always be connected to the same point in the circuit. This is usually done by connect- ing them together directly at the meter terminals. For measuring either AC or DC power, the common (t) junction is connected to either side of the power line. The voltage terminal (V) is connected to the opposite side of the power line. The current terminal (A) is connected to the power- consuming load resistance. 3-36 RESISTANCE IN AC CIRCUITS Wattmeters (continued) Wattmeters are not constructed with a D'Arsonval or Weston basic meter movement. Instead, they use a dynamometer type movement, which differs from the other types in that it has no permanent magnet to furnish the mag- netic field. This field is obtained from the field coils, two coils of wire placed opposite each other just as the poles of the permanent magnets are placed in other types of meters. These field coils are connected in series across the wattmeter current terminals, so that the circuit current flows through the coils when meas- urements are being made. A large circuit current makes the field coils act as strong magnets, while a small circuit current makes them act as weak magnets. Since the strength of the meter's magnetic field depends on the value of the circuit current, the wattmeter reading varies as the cir- cuit current varies. Since the current in the moving coil— the voltage coil— is dependent on the circuit voltage and this turning force is dependent on both the moving coil current and the field coil current, for a fixed current in the moving coil the turning force and meter reading depend only on the circuit current. 3-37 RESISTANCE IN AC CIRCUITS Wattmeters (continued) The moving coil of a wattmeter is like those used in the basic meter move- ment and is connected in series with an internal multiplier resistor to the voltage terminals of the wattmeter. The voltage terminals are connected across the circuit voltage in the same manner as a voltmeter, and the multiplier resistor limits the current flow through the moving coil. Since the resistance of the multiplier is fixed, the amount of current flowthrough it and the moving coil varies with the circuit voltage. A high voltage causes more current to flow through the multiplier and moving coil than a low voltage. For a given magnetic field, determined by the amount of circuit current (that which flows through the load), the turning force of the moving coil de- pends on the amount of current flowing through the moving coil. Since this current depends on the circuit voltage, the meter reading will vary as the circuit voltage varies. Thus, the meter reading depends on both the circuit current and the circuit voltage and will vary if either changes. Since power depends on both voltage and current, the meter measures power. Wattmeters may be used on DC, or on AC up to 133 cycles, but they must always be connected properly to prevent damage. When used on AC, the currents in the field coils and in the moving coil reverse simultaneously, so the meter turning force is always in the same direction. 3-38 RESISTANCE IN AC CIRCUITS Demonstration — Power in Resistive AC Circuits To show that effective values of AC voltage and current can be used to de- termine the power used in resistance circuits in the same manner as DC values, the instructor connects two lamp sockets in parallel across a 7.5- volt battery — five dry cells in series. Next the 0-10 volt DC voltmeter is connected across the lamp terminals to measure the circuit voltage. Six- volt lamps, each rated at 250 ma., are inserted in the sockets and you see that each lamp lights with equal brilliance. Together they allow 0.5 amperes to flow through the circuit, while the voltage is about 7.5 volts. Usingthe power formula (P = El), the power then is 7.5 x 0.5, or 3.75 watts. For an AC resistive circuit P =■ El - 3.75 watts Next the battery is disconnected and the DC voltmeter is replaced with an AC meter of the same range. The 6.3-volt transformer is used as an AC voltage source, and you see that the lamps light as brightly as they did in the DC circuit. Notice that the voltmeter reading is almost the same as that obtained using DC, about 7.5 volts. Applying the power formula, the effective AC power is 7.5 x 0.5, or 3.75 watts, equal to the DC power and causing the same amount of light. 3-39 RESISTANCE IN AC CIRCUITS Demonstration— Power in Resistive AC Circuits (continued) Wattmeters with a range of less than 75 watts are not generally available and, since it would be difficult to read 3 or 4 watts on a standard 0-75 watt meter scale, a larger amount of power is used to demonstrate power meas- urement with a wattmeter. To obtain a larger amount of power, the in- structor uses the 117-volt AC power line as a power source through a step- down autotransformer, which provides a voltage of about 60 volts AC. He will measure the power used by a resistor, first using a voltmeter and milliammeter and then a wattmeter. The instructor connects the DPST knife switch and the DP fuse holder in the line cord, as shown below, and inserts 1-amp fuses in the fuse holder. With a 0-500 ma. range AC milliammeter connected in series with one of its leads, the line cord is connected across a 150-ohm, 100-watt resistor. Then a 0-250 volt range AC voltmeter is connected directly across the terminals of the resistor to measure resistor voltage. The line cord plug is inserted in the transformer outlet, and with the switch closed, the line voltage indicated on the voltmeter is about 60 volts, and the 150-ohm re- sistor allows a current flow of about 0. 40 ampere as measured by the mil- liammeter. The resistor becomes hot due to the power being used, so the switch is opened as soon as the readings have been taken. The current reading may vary slightly as the heated resistor changes in resistance val- ue, so an average current reading is used. Computing the power used by the resistor you see that it is approximately 24 watts. Assuming that the voltage is 60 volts and the current is exactly 0.40 ampere, the power is then 60 x 0.40, or 24 watts. The actual results may be slightly different, depending on the exact voltage and current read- ings which are obtained. COMPUTING AC POWER USED BY A RESISTOR 3-40 RESISTANCE IN AC CIRCUITS Demonstration— Power in Resistive AC Circuits (continued) Now the milliammeter and voltmeter are removed from the circuit and the wattmeter is connected to measure directly the power used by the re- sistor. The current and voltage + terminals are connected together with a short j um per wire to form a common + terminal. One lead from the fuse block is then connected to this common ± terminal, and the other fuse block lead is connected to the remaining voltage terminal marked V. Wires are connected to each end of the resistor and these are in turn connected to the wattmeter — one to the voltage terminal V and the other to the cur- rent terminal A. When the connections are completed, the autotransformer is connected to the AC power outlet and the switch is closed. You see that the wattmeter indicates that about 24 watts of power is being used. The wattage reading will vary slightly as the resistor heats and changes value, but will become steady when the resistor temperature reaches a maximum. Observe that the measured power is very nearly the same as that obtained using a volt- meter and a milliammeter; and the two results can be considered to be equal for all practical purposes. 3-41 RESISTANCE IN AC CIRCUITS Review of Resistance in AC Circuits Suppose you review some of the facts concerning AC power, power waves and power in resistive circuits. These facts you have already learned will help you to understand other AC circuits, which are not made up of only pure resistance. AC POWER WAVE — Pictorial graph of all the values of in- stantaneous power. + Power AVERAGE POWER - A value equal to the axis of symmetry drawn through a power wave. WATTMETER — Meter used to measure power directly when connected in a circuit. Remember that the power formula (P = El) can be used to find the power used in a resistive AC circuit, provided effective values of E and I are used. INDUCTANCE IN AC CIRCUITS Emf of Self-Induction Inductance exists in a circuit because an electric current always produces a magnetic field. The lines of force in this field always encircle the con- ductor which carries the current, forming concentric circles around the conductor. The strength of the magnetic field depends on the amount of current flow, with a large current producing many lines of force and a small current producing only a few lines of force. Current produces magnetic field Amount of current flow determines strength of magnetic field Small Small Increased current magnetic current Larger magnetic flow field ' flow field .When the circuit current increases or decreases, the magnetic field strength increases and decreases in the same direction. As the field strength increases, tl)e lines of force increase in number and expand out- ward from the center of the conductor. Similarly, when the field strength decreases, the lines of force contract toward the center of the conductor. It is actually this expansion and contraction of the magnetic field as the current varies which causes an emf of self-induction, and the effect is known as "inductance. " IC FIELD EXPANDS AND CONTRACTS WITH VARYING CURRENT . . . Conductor * + magnet! c A A V Current flow through conductor Decreasing current flow Maximum current flow EMF OF SELF-INDUCTION current flow E EFFECT KNOWN AS CAUSING INDUCTANCE IN AC CIRCUITS Inductance in a DC Circuit To gee how inductance is caused, suppose your circuit contains a coil like the one shown below. As long as the circuit switch is open, there is no current flow and no field exists around the circuit conductors. When the switch is closed, current flows through the circuit and lines of force expand outward around the circuit conductors including the turns of the coil. At the instant the switch is closed, the current flow starts rising from zero toward its maximum value. Although this rise in current flow is very rapid, it cannot be instantaneous. Imagine that you actually are able to see the lines of force in the circuit at the instant the current starts to flow. You see that they form a field around the circuit conductors. The lines of force around each turn continue their expansion and, in so do- ing, cut across adjacent turns of the coil. This expansion continues as long as the circuit current is increasing, with more and more lines of force from the coil turns cutting across adjacent turns of the coil. 3-45 INDUCTANCE IN AC CIRCUITS Inductance in a DC Circuit (continued) Whenever a magnetic field moves across a wire, it induces an emf in the wire. Whenever a current flows through a coil, it induces a magnetic field that cuts adjacent coil turns. Whenever the initial current changes in di- rection, the induced field changesr and the effect of this changing field in cutting the adjacent coil turns is to oppose the change in current. The ini- tial current change is caused by the emf, or voltage, across the coil and this opposing force is an emf of self-induction. Inductance is the property of generating an emf of self-induction which opposes changes in the coil. Effect of Emf of Self-Induction _ Opposition to - current change Current flow No Emf of Expanding Magnetic Field Increasing Current Fixed Magnetic Field A. Change in Current Generates An Current Constant Induced Emf When the circuit current reaches its maximum value, determined by the circuit voltage and resistance, it no longer changes in value and the field no longer expands, so that no emf of self-induction is generated. The field remains stationary but, should the current attempt to rise or fall, the field will either expand or contract and generate an emf of self-induction op- posing the change in current flow. For direct current, inductance affects the current flow only when the power is turned on and off, since only at those times does the current change in value. 3-46 INDUCTANCE IN AC CIRCUITS Inductance in a DC Circuit (continued) With the current and magnetic field at maximum, no emf of self-induction is generated but if you lowered the source voltage or increased the circuit resistance, the current would decrease. Suppose the source voltage de- creases. The current drops toward its new Ohm's law value, determined by E and R. As the current decreases the field also diminishes, with each line of force contracting inward toward the conductor. This contracting or collapsing field cuts across the coil turns in a direction opposite to that caused by the rise in circuit current. Since the direction of change is reversed, the collapsing field generates an emf of self-induction opposite to that caused by the expanding field, thus having the same polarity as the source voltage. This emf of self-induction then increases the source voltage, trying to prevent the fall in current. However, it cannot keep the current from falling indefinitely since the emf of self-induction ceases to exist whenever the current stops changing. Thus inductance — the effect of emf of self-induction — opposes any change in cur- rent flow, whether it be an increase or decrease, slowing down the rate at which the change occurs. A COLLAPSING FIELD ALSO GENERATES AN EMF OF SELF-INDUCTION j|| Effect of Emf of Self-Induction Decreasing Current Collapsing Magnetic Field Effect of Emf of Self-Induction THE EMF OF SELF-INDUCTION TRIES TO KEEP THE CURRENT FROM DECREASING Opposition to current change caused by emf of self-induction 3-47 INDUCTANCE IN AC CIRCUITS Inductance in a DC Circuit (continued) As long as the circuit is closed, the current remains at its Ohm's law value and no induced emf is generated. Now suppose you open the switch to stop the current flow. The current should fall to zero and stop flowing immedi- ately but, instead, there is a slight delay and a spark jumps across the switch contacts. When the switch is opened, the current drops rapidly toward zero and the field also collapses at a very rapid rate. The rapidly collapsing field gen- erates a very high induced emf, which not only opposes the change in cur- rent but also causes an arc across the switch to maintain the current flow. Although only momentary, the induced emf caused by this rapid field col- lapse is very high, sometimes many times that of the original source volt- age. This action is often used to advantage in special types of equipment to obtain very high voltages. Caused By Strong Induced Emf 3-48 INDUCTANCE IN AC CIRCUITS Inductance Symbols While you cannot see inductance, it is present in every electrical circuit and has an effect on the circuit whenever the circuit current changes. In electrical formulas the letter L is used as a symbol to designate induct- ance. Because a coil of wire has more inductance than a straight length of the same wire, the coil is called an "inductor." Both the letter and the symbol are illustrated below. Since direct current is normally constant in value except when the circuit power is turned on and off to start and stop the current flow, inductance affects DC current flow only at these times and usually has very little ef- fect on the operation of the circuit. Alternating current, however, is con- tinuously changing so that the circuit inductance affects AC current flow at all times. Although every circuit .has some inductance, the value de- pends upon the physical construction of the circuit, and the electrical de- vices used in it. In some circuits the inductance is so small its effect is negligible, even for AC current flow. INDUCTANCE IN AC CIRCUITS Factors Which Affect Inductance Every complete electric circuit has some inductance since the simplest circuit forms a complete loop or single-turn coil. An induced emf is gen- erated even in a straight piece of wire, by the action of the magnetic field expanding outward from the center of the wire or collapsing inward to the wire center. The greater the number of adjacent turns of wire cut across by the expanding field, the greater the induced emf generated — so that a coil of wire having many turns has a high inductance. 3-50 INDUCTANCE IN AC CIRCUITS Factors Which Affect Inductance (continued) Any factors which tend to affect the strength of the magnetic field also affect the inductance of a circuit. For example, an iron core inserted in a coil increases the inductance because it provides a better path for mag- netic lines of force than air. Therefore, more lines of force are present that can expand and contract when there is a change in current. A copper core piece has exactly the opposite effect. Since copper opposes lines of force more than air, inserting a copper core piece results in less field change when the current changes, thereby reducing the inductance. 3-51 INDUCTANCE IN AC CIRCUITS Units of Inductance In electrical formulas the letter L is used as a symbol to designate in- ductance. The basic unit of measure for inductance is the henry. For quantities of inductance smaller than one henry, the millihenry and microhenry are used. A unit larger than the henry is not used since inductance normally is of a value which can be expressed in henries or part of a henry. Inductance can only be measured with special laboratory instruments and depends entirely on the physical constiuction of the circuit. Some of the factors most important in determining the amount of inductance of a coil are: the number of turns, the spacing between turns, coil diameter, kind of material around and inside the coil, the wire size, number of layers of wire, type of coil winding and the overall shape of the coil. Wire size does not affect the inductance directly, but it does determine the number of turns that can be wound in a given space. All of these factors are variable, and no single formula can be used to find inductance. Many differently con- structed coils could have an inductance of one henry, and each would have the same effect in the circuit. ^(tcUictcuice defected^ m,. , , , ►THE NUMBER OF TURNS ► THE CORE MATERIAL IRON AIR COPPER ► SPACING BETWEEN TURNS WIRE SIZE qmp rjn ► OVERALL SHAPE OF COIL -NUMBER OF LAYERS OF WINDINGS 3-52 INDUCTANCE IN AC CIRCUITS Mutual Induction The term "mutual induction" refers to the condition in which two circuits are sharing the energy of one of the circuits. It means that energy is being transferred from one Circuit to another. Consider the diagram below. Coil A is the primary circuit which obtains energy from the battery. When the switch is closed, the current starts to flow and a magnetic field expands out of coil A. Coil A then changes electrical energy of the battery into the magnetic energy of a msgnetic field. When the field of coil A is expanding, it cuts across coil B, the secondary circuit, inducing an emf in coil B. The indicator (a galvanometer) in the secondary circuit is deflected, and shows that a current, developed by the induced emf, is flowing in the circuit. The induced emf may be generated by moving coil B through the flux of coil A. However, this voltage is induced without moving coil B. When the switch in the primary circuit is open, coil A has no current and no field. As soon as the switch is closed, current passes through the coil and the magnetic field is generated. This expanding field moves or "cuts" across the wires of coil B, thus inducing an emf without the movement of coil B. The magnetic field expands to its maximum strength and remains constant as long as full current flows. Flux lines stop their cutting action across the turns of coil B because expansion of the field has ceased. At this point the indicator needle reads zero because no induced emf exists any- more. If the switch is opened, the field collapses back to the wi res of coil A. As it does so, the changing flux cuts across the wires of coil B, but in the opposite direction. The current present in the coil causes the indica- tor needle to deflect, showing this new direction. The indicator, then, shows current flow only when the field is changing, either building up or collapsing. In effect, the changing field produces an induced emf exactly as does a magnetic field moving across a conductor. This principle of in- ducing voltage by holding the coils steady and forcing the field to change is used in innumerable applications. Hie transformer, as shown In the dia- gram below, is particularly suitable for operation by mutual induction. For purposes of explanation a battery is used in the above example. The transformer is, however, a perfect component for transferring and changing AC voltages as needed. Mutual Induction Circuits 3-53 INDUCTANCE IN AC CIRCUITS How a Transformer Works When AC flows through a coil, an alternating magnetic field is generated around the coil. This alternating magnetic field expands outward from the center of the coil and collapses into the coil as the AC through the coil varies from zero to a maximum and back to zero again. Since the alter- nating magnetic field must cut through the turns of the coil an emf of self induction is induced in the coil which opposes the change in current flow. EMF OF SELF INDUCTION Field expansion n Field contraction M AC current flow dC> Opposition to current flow offered by counter -emf If the alternating magnetic field generated by one coil cuts throujgh the turns of a second coil, an emf will be generated in this second coil just as an emf is induced in a coil which is cut by its own magnetic field. The emf generated in the second coil is called the "emf of mutual induction," and the action of generating this voltage is called "transformer action." In transformer action, electrical energy is transferred from one coil (the primary) to another (the secondary) by means of a varying magnetic field. Magnetic lines Expanding Field Collapsing Field 3-54 INDUCTANCE IN AC CIRCUITS How a Transformer Works (continued) A simple transformer consists of two coils very close together, electri- cally insulated from each other. The coil to which the AC is applied is called the "primary." It generates a magnetic field which cuts through the turns of the other coil, called the "secondary," and generates a voltage in it. The coils are not physically connected to each other. They are, how- ever, magnetically coupled to each other. Thus, a transformer transfers electrical power from one coil to another by means of an alternating mag- netic field. 3 Assuming that all the magnetic lines of force from the primary cut through all the turns of the secondary, the voltage induced in the secondary will de- pend on the ratio of the number of turns in the secondary to the number of turns in the primary. For example, if there are 1000 turns in the sec- ondary and only 100 turns in the primary, the voltage induced in the sec- ndary will be 10 times the voltage applied to the primary ( j^y = 10). Since t': > are more turns in the secondary than there are in the primary, the - •< rmer is called a "step-up transformer." If, on the other hand, the sect ary has 10 turns and the primary has 100 turns, the voltage induced m i() he S j COndary wil1 be one -tenth of the voltage applied to the primary (l00 = Iq)’ Since there are less turns in the secondary than there are in 3-55 INDUCTANCE IN AC CIRCUITS How a Transformer Works (continued) The current in the primary of a transformer flows in a direction opposite to that which flows in the secondary because of the emf of mutual-induction. An emf of self-induction is also set up in the primary which is in opposition to the applied emf. When no load is present at the output of the secondary, the primary current is very small because the emf of self-induction is almost as large as the applied emf. If no load is present at the secondary there is no current flow. Thus, the magnetic field of self-induction, which usually bucks the mag- netic field of the primary, cannot be developed in the secondary. The mag- netic field of the primary may then develop to its maximum strength with- out opposition from the field which is usually developed by current flow in the secondary. When the primary field is developing to its maximum strength it produces the strongest possible emf at self-induction and this opposes the applied voltage. This is the point, mentioned above, at which the emf of self-induction almost equals the applied emf. Any difference between the emf of self-induction and the applied emf causes a small cur- rent to flow in the primary and this is the exciting or magnetizing current. The current which flows in the secondary is opposite to the current in the primary. As a load is applied to the secondary it causes the momentary collapse of flux lines which produces a demagnetizing effect on the flux linking the primary. The reduction in flux lines reduces the emf of self- induction and permits more current to flow in the primary. In all cases of electromagnetic induction the direction of the induced emf is such that the magnetic field set up by the resulting current opposes the motion which is producing the emf. This is a statement of Lenz's law which you will learn about in the next section. In order to find the unknowns in a transformer use the formula *s Tp -=r- = t - = V an d cross-multiply to find the required information. Further “s T> x s details about transformers will be included at the end of this section. 2 To 1 Ratio 100 Turns 50 Turns Example of a Step-Down Transformer 3-56 INDUCTANCE IN AC CIRCUITS Faraday's Law Michael Faraday was an English scientist who did a great deal of important work in the field of electromagnetism. He is of interest to you at this time because his work in mutual induction eventually led to the development of the transformer. Faraday is responsible for the law which is used in developing the princi- ples of mutual induction. He found that if the total flux linking a circuit changes with time, an emf is induced in the circuit. Faraday also found that if the rate of flux-change is increased, the magnitude of the induced emf is increased as well. Stated in other terms, Faraday found that the character of an emf induced in a circuit depends upon the amount of flux and also the rate of change of flux which links a circuit. You have all seen demonstrations of the principle just stated. You have been shown that if a conductor is made to move with respect to a magnetic field an emf is induced in the conductor which is directly proportional to the velocity of the conductor with respect to the field. The other point concerning Faraday's Law which has been demonstrated is the fact that the voltage induced in a coil is proportional to the number of turns of the coil, the magnitude of the inducing flux and the rate of change of this flux. An example of mutual induction (inducing an emf in a neighboring conductor) is now described. Consider the two coils in the figure below. Electrons are moving as a current in the directions indicated. This current produces a flux of magnetic field and if the current remains constant the number of flux lines produced is fixed. If, however, the current is changed by open- ing the shorting switch, the number of flux lines in coil A is decreased, and consequently the flux linking coil B is decreased also. This changing flux induces an emf in coil g, as evidenced by the movement of the indi- cator pointer. Thus, it is seen that energy can be transferred from one circuit to another by the principle of electromagnetic induction. A battery is used in the above diagram as a source of emf. The only way current variations can be developed, then, is by the opening or closing of the switch. If an AC voltage source with an extremely low frequency (one cycle per second) is used to replace the battery, the indicator shows con- tinuous variations in current. The indicator needle moves to the left (or right) first, and then reverses its position, to show the reversal in AC flow. 3-57 INDUCTANCE IN AC CIRCUITS Inductive Time Constant in a DC Circuit In a circuit consisting of a battery, switch and a resistor in series, the current rises to its maximum value at once whenever the switch is closed. Actually it cannot change from zero to its maximum value instantaneously, but the time is so short that it can be considered to be instantaneous. If a coil of wire is used in series with the resistor, the current does not rise instantaneously— it rises rapidly at first, then more slowly as the maxim um value is approached. For all inductive circuits the shape of this curve is basically the same, although the total time required to reach the maximum current value varies. The time required for the current to rise to its maximum value is determined by the ratio of the circuit inductance to its resistance in ohms. This ratio L/R— inductance divided by the resistance— is called the "time constant" of the inductive circuit and gives the time in seconds required for the circuit current to rise to 63.2 percent of its maximum value. This delayed rise in the current of a circuit is called "self-inductance," and is used in many practical circuits such as time-delay relay and starting circuits. SWITCH CLOSED i Ohm s Law value i i 1 oi current ^ \ 63.2' , of maximum 4: .t INDUCTANCE R -wm TIME CONSTANT IS THE TIME REQUIRED FOR THE CURRENT TO REACH 63.2% OF ITS MAXIMUM VALUE. TIME CONSTANT EQUALS L/R L RESISTANCE L L L DELAYED RISE OF CURRENT IN AN INDUCTIVE CIRCUIT 3-58 If the coil terminals are shorted together at the same moment that the bat- tery switch is opened, the coil current continues to flow due to the action of the collapsing field. The current falls in the same manner as the origi- nal rise in current, except that the curve is in the opposite direction. Again the "time constant" can be used to determine when the current has decreased by 63.2 percent, or has reached 36.8 percent of its original max- imum value. For inductive circuits the lower the circuit resistance, the longer the time constant for the same value of inductance. 3-59 INDUCTANCE IN AC CIRCUITS Inductive Time Constant in a DC Circuit (continued) The time constant of a given inductive circuit is always the same for both the build-up and decay of the current. If the maximum current value dif- fers, the curve may rise at a different rate but will reach its maximum in the same amount of time; and the general shape of the curve is the same. Thus, if a greater voltage is used, the maximum current will in- crease but the time required to reach the maximum is unchanged. Every inductive circuit has resistance, since the wire used in a coil always has resistance. Thus a perfect inductance— an inductor with no resistance — is not possible. Ill ■HR Of h Ohm’s law value | of current ^--JT 63.2% MAX. TIME CONSTANT TIME i i Ohms law value of current i w 63.2% MAX. I n 0 TIME CONSTANT TIME RESISTANCE INCREASED VOLTAGE 3-60 INDUCTANCE IN AC CIRCUITS Inductive Reactance Inductive reactance is the opposition to current flow offered by the induct- ance of a circuit. As you know, inductance only affects current flow while the current is changing, since the current change generates an induced emf. For i irect current the effect of inductance is noticeable only when the cur- rent is turned on and off but, since alternating current is continuously changing, a continuous induced emf is generated. Suppose you consider the effect of a given inductive circuit on DC and AC waveforms. The time constant of the circuit is always the same, deter- mined only by the resistance and inductance of the circuit. For DC the current waveforms would be as shown below. At the beginning of the current waveform, there is a shaded area between the maximum cur- rent value and the actual current flow which shows that inductance is op- posing the change in current as the magnetic field builds up. Also, at the end of the current waveform, a similar area exists showing that current flow continues after the voltage drops to zero because of the field collapse. These shaded areas are equal, indicating that the energy used to build up the magnetic field is given back to the circuit when the field collapses. 3-61 INDUCTANCE IN AC CIRCUITS Inductive Reactance (continued) The same inductive circuit would affect AC voltage and current waveforms as shown below. The current rises as the voltage rises, but the delay due to inductance prevents the current from ever reaching its maximum DC value before the voltage reverses polarity and changes the direction of current flow. Thus, in a circuit containing inductance, the maximum cur- rent will be much greater for DC than for AC. AC VOLTAGE AND I CURRENT WAVEFORMS IN AN INDUCTIVE CIRCUIT If the frequency of the AC wave is low, the current will have time to reach a higher value before the polarity is reversed than if the frequency is high. Thus the higher the frequency, the lower the circuit current through an in- ductive circuit. Frequency, then, affects the opposition to current flow as does circuit inductance. For that reason, inductive reactance— opposition to current flow offered by an inductance— depends on frequency and induct- ance. The formula used to obtain inductive reactance is Xl = 2fTfL. In this formula X L is inductive reactance, f is frequency in cycles per second L is the inductance in henries, and 2 is a constant number (6.28) representing one complete cycle. Since Xl represents opposition to cur- rent flow, it is expressed in ohms. 3-62 INDUCTANCE IN AC CIRCUITS Inductive Reactance (continued) Actually the circuit current does not begin to rise at the same time the voltage begins to rise. The current is delayed to an extent depending on the amount of inductance in the circuit as compared to the resistance. If an AC circuit has only pure resistance, the current rises and'falls at ex- actly the same time as the voltage and the two waves are said to be in phase with each other. With a theoretical circuit of pure inductance and no resistance, the current will not begin to flow until the voltage has reached its maximum value, and the current wave then rises while the voltage falls to zero. At the moment the voltage reaches zero the current starts to drop towards zero, but the collapsing field delays the current drop until the voltage reaches its max- imum value in the opposite polarity. This continues as long as voltage is applied to the circuit, with the voltage wave reaching its maximum value a quarter cycle before the current wave on each half cycle. A complete cy- cle of an AC wave is considered to be 360 degrees, represented by the emf generated in a wire rotated once around in a complete circle between two opposite magnetic poles. A quarter cycle then is 90 degrees: and in a purely inductive circuit the voltage wave leads the current by 90 degrees or, in opposite terms, the current wave lags the voltage by 90 degrees. currents ARE 90 n»v PHASE 1 7 a CIRCUIT OF PURE INDUCTANCE f : VOLTAGE SOURCE L ii § o o 3-63 INDUCTANCE IN AC CIRCUITS Inductive Reactance (continued) In a circuit containing both inductive reactance and resistance, the AC cur- rent wave will lag the voltage wave by an amount between zero degrees and 90 degrees; or, stated otherwise, it will lag somewhere between"in phase" and "90 degrees out of phase." The exact amount of lag depends on the ratio of circuit resistance to inductance— the greater the resistance com- pared to the inductance, the nearer the two waves are to being in phase ; and the lower the resistance compared to the inductance, the nearer the waves are to being a full quarter cycle (90 degrees) "out of phase. When stated in degrees the current lag is called the "phase angle." If the phase angle between the voltage and the current is 45 degrees lagging, it means that the current wave is lagging the voltage wave by 45 degrees. Since thisds halfway between zero degrees— the phase angle for a pure re- sistive circuit— and 90 degrees— the phase angle for a pure inductive cir- cuit— the resistance and the inductance reactance must be equal, with each having an equal effect on the current flow. 3-34 INDUCTANCE IN AC CIRCUITS Demonstration— Effect of Core Material on Inductance The instructor wires a series circuit of the flat air-core coil and the 60- watt lamp. When the circuit is energized from the 115-volt AC line, the lamp brilliance is noted. With the circuit energized, the instructor carefully inserts the iron core into the coil. Note the decrease in lamp brilliance resulting from the in- creased inductance of the coil. A larger percentage of the 115-volt source voltage is now dropped across the coil. Next he removes the iron core and inserts a copper core. Note the in- c re as e in lamp brilliance resulting from the decreased inductance of the f.°, : A, he 1 , arge edd y cur rent losses in the copper weaken the coil magnetic field, thus decreasing its inductance. A larger percentage of the source voltage is now dropped across the lamp and it, therefore, gets brighter. He next removes the copper core and inserts the laminated core. Note that the lamp brilliance has dropped greatly. The laminated iron core has increased the coil inductance an even greater amount than the solid iron core because the laminations have greatly reduced the hysteresis losses. Most of the source voltage is now dropped across the coil and as a result the lamp barely lights. 60-watt clear red lamp Iron core core core HOW INDUCTANCE VARIES WITH CORE MATERIAL 3-65 INDUCTANCE IN AC CIRCUITS Demonstration — Generation of Induced EMF When the current flow in a DC circuit containing inductance is stopped abruptly, by opening a switch, for example, the magnetic field of the induct- ance tries to collapse instantaneously. The rapid collapse of the field mo- mentarily generates a very high voltage, and this induced emf may cause an arc at the switch. While the field collapse is too rapid to allow meas- urement of this voltage with a voltmeter, a neon lamp can be used to show that the voltage is much higher than the original battery voltage. Neon lamps differ from ordinary lamps in that they require a certain volt- age before they begin to light. This voltage, called the "starting voltage," varies for different neon lamps. Its value can be determined by increasing the voltage applied across the lamp until it lights. The voltage applied at the time the lamp first lights is the starting voltage. To find the starting voltage required for the neon lamp, the instructor first connects two 45-volt batteries in series to form a 90-volt battery. Across the 90-volt battery he connects a variable resistor as a potentiometer, with the outside or end terminals of the variable resistor connected to the bat- tery terminals. A lamp socket is connected between the center terminal of the variable resistor and one of the outside terminals, and a 0-100 volt range DC voltmeter is connected across the lamp socket terminals. With the neon lamp inserted, the instructor varies the voltage applied to the lamp by varying the setting of the variable resistor. The correct start- ing voltage is found by lowering the voltage to a value which does not light the lamp, and then slowly increasing it until the lamp first lights. You see that the starting voltage required to light the lamp is approximately 70 volts, 3-66 INDUCTANCE IN AC CIRCUITS Demonstration— Generation of Induced Emf (continued) Next, four dry cells are connected in series to form a 6- volt battery, with the lamp socket connected across its terminals through a fuse and switch. A neon lamp is inserted in the socket, and the choke coil is connected across the lamp terminals. w hen the instructor closes the switch, you see that the lamp does not light and the battery voltage measured with an 0-10 volt DC voltmeter is 6 volts. Smce six volts is less than the starting voltage of the lamp, some means of obtaining a higher voltage is required in order to cause the lamp to light. As the switch is opened you see that the lamp flashes, indicating that the voltage across the lamp hnd coil in parallel is higher than the starting volt- age required for the lamp. This voltage is the induced emf generated by the collapsing field of the choke, and is a visible effect of inductance. 3-67 INDUCTANCE IN AC CIRCUITS Demonstration — Current Flow in Inductive Circuits To compare the effect of circuit inductance on the amount of current flow in AC and DC circuits, the instructor connects two identical circuits— one using the six-volt battery as a DC voltage source and the other using the 6.3-volt transformer as an AC voltage source, with the correct type of meters (AC or DC) being used in each circuit. At first the two circuits will be compared when 60 ohms of resistance is the only load, with 0-500 ma. range milliammeters, 0-10 volt DC and 0-25 volt AC voltmeters con- nected to measure the voltage and current. Two 30-ohm resistors in se- ries are used to obtain each resistance. Observe that the current and volt- age readings are very nearly the same for the two circuits. RESISTANCE HAS THE SAME EFFECT ON AC AND DC CURRENT FLOW 3-68 INDUCTANCE IN AC CIRCUITS Demonstration — Current Flow in Inductive Circuits (continued) OBSERVING AC AND DC CURRENT FLOW IN AN INDUCTIVE CIRCUIT DC AC 5-henry Filter Choke Inductance holds back AC current more than DC current Next, the resistors are removed from the circuits and replaced by 5-henry, 60-ohm filter chokes. With power applied, you see that the current flow in the DC circuit is approximately the same as when the resistors were in the circuit, but the current in the AC circuit is much less and cannot be read on the 0-500 ma. range AC milliammeter, because the deflection is too small to be observed. Although the filter choke is rated at 2 henries, it only operates at this value when the current is 200 ma. DC. Its inductance is greater for the smaller current values which you commonly use, and its effect can be calculated by assuming an inductance of 5 henries. For DC, the inductance has no effect, and the choke merely acts as a 60-ohm resistor. For AC, since the volt- age and current are changing constantly, inductance is an important factor. The effect which inductive reactance has on AC can be calculated by using the formula Xl = 2fffL (27 t = 6.28, f = 60 cycles which is the power line frequency and L = 5 henries). You can find the inductive reactance, Xl, by substituting these values for the formula symbols and multiplying them together (Xl = 6.28 x 60 x 5 = 1884 O). Inductive reactance is expressed in ohms, since it opposes or "resists" AC current flow. 3-69 INDUCTANCE IN AC CIRCUITS Demonstration — Current Flow in Inductive Circuits (continued) To further demonstrate that it is the inductive reactance which is reducing the current flow in the AC circuit, the instructor connects a lamp socket in series with the choke in each circuit. With the power applied to each cir- cuit you see that the lamp lights dimly in the DC circuit, but the current in the AC circuit is insufficient to light the lamp. Using short pieces of wire as jumpers, the instructor shorts across the terminals of the chokes in each circuit. In the DC circuit the lamp bright- ness increases, showing that the circuit resistance has been reduced. In the AC circuit the lamp lights to a brightness equal to that of the lamp in the DC circuit. Since the lamp brightness is changed from no light to max- imum brightness in the AC circuit, you see that the choke or inductance has a great effect on the current in the AC circuit, while in the DC circuit it merely acts as a resistance. HOW INDUCTIVE REACTANCE AFFECTS THE TOTAL CIRCUIT CURRENT 3-70 INDUCTANCE IN AC CIRCUITS Review of Inductance in AC Circuits To review inductance, what it is and how it affects current flow, consider facts concerning inductance and inductive reactance. INDUCTANCE — The property of a circuit which opposes any change in the current flow; measured in henries and symbolized by letter L. INDUCTOR — A coil of wire used to supply inductance in a circuit. INDUCED EMF — A voltage which is gen- erated within a circuit by the movement of the magnetic field whenever the circuit cur- rent changes, and which opposes the cur- rent change. INDUCTIVE TIME CONSTANT — The ratio of L to R which gives the time in seconds required for the circuit current to rise to 63.2 percent of its maximum value. INDUCTIVE REACTANCE — The action of inductance in opposing the flow of AC cur- rent and in causing the current to lag the voltage; measured in ohms and symbolized by letter Xl- PHASE ANGLE — The amount in degrees by which the current wave lags the voltage wave. TRANSFORMER ACTION — The method of transferring electrical energy from one coil to another by means of an alternating mag- netic field. The coil which generates the magnetic field is called the primary, and the coil in which the voltage is induced is called the secondary. The voltage induced in the secondary depends upon the turns ratio between the secondary and primary. 3-71 POWER IN INDUCTIVE CIRCUITS The Effect of Phase Difference on the Power Wave In a theoretical circuit containing only pure inductance the current lags the voltage by 90 degrees. To determine the power wave for such a cir- cuit, all of the corresponding instantaneous values of voltage and current are multiplied together to find the instantaneous values of power, which are then plotted to form the power curve. As you already know the power curve for "in phase" voltages and currents is entirely above the zero axis, since the result is positive when either two positive numbers are multiplied together or two negative numbers are multiplied together. When a negative number is multiplied by a positive number however, the result is a negative number. Thus, in computing in- stantaneous values of power when the current and voltage are not in phase, some of the values are negative. If the phase difference is 90 degrees, as in the case of a theoretical circuit containing only pure inductance, half the instantaneous values of power are positive and half are negative as shown below. For such a circuit the voltage and current axis is also the power wave axis, and the frequency of the power wave is twice that of the current and voltage waves. POWER IN A CIRCUIT CONTAINING wcUtctcutce. 0*ttcf 3-72 POWER IN INDUCTIVE CIRCUITS Positive and Negative Power That portion of a power wave which is above the zero axis is called "pos- itive power" and that which is below the axis is called "negative power." Positive power represents power furnished to the circuit by the power source, while negative power represents power the circuit returns to the power source. In the case of a pure inductive circuit the positive power furnished to the circuit causes a field to build up. When this field collapses, it returns an equal amount of power to the power source. Since no power is used for heat or light in a circuit containing only pure inductance (if it were possible to have such a circuit), no actual power would be used even though the current flow were large. The actual power used in any circuit is found by subtracting the negative power from the positive power. POSITIVE AND NEGATIVE POWER 3-73 POWER IN INDUCTIVE CIRCUITS Apparent and True Power Any practical inductive circuit contains some resistance, and since the phase angle depends on the ratio between the inductive reactance and the resistance, it is always less than 90 degrees. For phase angles of less than 90 degrees the amount of positive power always exceeds the negative power, with the difference between the two representing the actual power used in overcoming the circuit resistance. For example, if your circuit contains equal amounts of inductive reactance and resistance, the phase angle is 45 degrees and the positive power exceeds the negative power, as shown below. 90° Phase Angle (Negative power equals the positive power) 45° Phase Angle (Positive power exceeds negative power) Apparent power Exl = V. A. Power wave axis — True power used (I^R) TR P. F. = — — = 70% at 45° V. A. DECREASING THE PHASE ANGLE INCREASES THE TRUE POWER The average value of actual power, called "true power," is represented by an axis drawn through the power wave halfway between the opposite max- imum values of the wave. As the phase angle increases, this axis moves nearer to the axis for voltage and current. In AC circuits the apparent power is found by multiplying the voltage and current just as in DC cir- cuits (Apparent power = Voltage x Current). When apparent power is di- vided into true power, the resultant decimal is the power factor. Apparent power and true power for AC circuits are equal only when the cir- cuit consists entirely of pure resistance. The difference between apparent and true power is sometimes called "wattless power" since it does not pro- duce heat or light but does require current flow in a circuit. 3-74 POWER IN INDUCTIVE CIRCUITS Power Factor You have already learned that in a pure resistive circuit I 2 R or E 2 /R (power in watts) is equal to Ieff Eeff (apparent power), and that the power factor is equal to 100%. Power factor is the ratio of true power to apparent power. In an inductive circuit a phase angle exists and power in watts does not equal apparent power, and as a result power factor will be between zero and 100%. POWER FACTOR IN A PURE INDUCTIVE CIRCUIT Ieff x E e ff = Apparent Power (VA) = 1000 I^R or E^/R = True Power (Watts) = 0 p F True Power Watts 0 Apparent Power " Volt Amps " 1000 ~ OT 'o Power Factor is a method of determining what percentage of the supplied power is used in watts, and what percentage is returned to the source as wattless power. Power Factor in a pure induc^ve circuit is equal to zero percent. The phase angle is 90°. 3-75 POWER IN INDUCTIVE CIRCUITS Measurement of True Power Since the product of the circuit current and voltage is the apparent power and not the true power, a wattmeter is used to measure the true power used in an AC circuit. Voltmeter and ammeter readings are not affected by the phase difference between circuit current and voltage, since the voltmeter reading is affected only by voltage and the ammeter reading is affected only by current. The wattmeter reading is affected by both the circuit current and voltage and the phase difference between them, as shown below. When the voltage and current are in phase, the current increases at the same time as the voltage. The circuit current increases the meter field simultaneously with the increase in current through the moving coil which is caused by the voltage. The voltage and current thus act together to in- crease the turning force on the meter pointer. IN-PHASE VOLTAGE AND CURRENT ACT TOGETHER TO INCREASE THE WATTMETER READING Terminals connected 1 and E in same direction Increasing current increases magnetic field Increasing voltage , increases A. a. turning lorcel^KjJ m W * If the current lags the voltage, the meter field does not increase at the same time as the moving coil current. This results in less turning force on the wattmeter pointer. The power indicated then is less than with in- phase voltage and current of the same magnitude. OUT-OF-PHASE VOLTAGE AND CURRENT ACT IN OPPOSITION, DECREASING n THE WATTMETER READING Increasing current increases turning force Decreasing voltage decreases turning force! Terminals connected 1 and E in same direction Similarly, if the current leads the voltage, the meter field strength and the moving coil current will not increase at the same time. This results in a lower wattmeter reading, the actual power used by the circuit again being less than the apparent power. 3-76 CAPACITANCE IN AC CIRCUITS Capacitance When the voltage across an electric circuit changes, the circuit opposes this change. This oppositioji is called "capacitance." Like inductance, capacitance cannot be seen, but its effect is present in every electrical circuit whenever the circuit voltage changes. Capacitance Opposes Any Change in Circuit Voltage Because DC voltage usually varies only when it is turned on and off, ca- pacitance affects DC circuits only at these times. In AC circuits, however, the voltage is continuously changing, so that the effect of capacitance is continuous. The amount of capacitance present in a circuit depends on the physical construction of the circuit and the electrical devices used. The capacitance may be so small that its effect on circuit voltage is neglibible. Electrical devices which are used to add capacitance to a circuit are called "capacitors," and the circuit symbol used to indicate a capacitor is shown below. Another term frequently used instead of capacitor is "condenser." You will often find the words "capacitor" and "condenser" used inter- changeably, but they mean the same thing. Symbols for condensers and capacitors HhHf 3-77 CAPACITANCE IN AC CIRCUITS Capacitance (continued) Capacitance exists in an electric circuit because certain parts of the cir- cuit are able to store electric charges. Consider two flat metal plates placed parallel to each other, but not touching. You have already found out — while working with static electricity — that these plates can be charged either positively or negatively, depending on the charge which you transfer to them. If charged negatively, a plate will take extra electrons, but if charged positively it will give up some of its electrons. Thus the plates may have either an excess or lack of electrons. The plates may be charged independently with either plate being charged positively, negatively, or having no charge. Thus they may both have no charge, one plate only may be charged, both plates may have the same type of charge, or the two plates may have opposite charges. 3-78 CAPACITANCE IN AC CIRCUITS Capacitance (continued) In order to charge the plates, an electrical force is required. The greater the charge to be placed on each plate, the greater the electrical force re- quired. For example, to charge a plate negatively, you must force extra electrons onto it from a source of negative charge. The first few extra electrons go 1 onto the plate easily but once there, they oppose or repel any other electrons which try to follow them. As more electrons are forced onto the plate, this repelling force increases, so that a greater force is required to move additional electrons. When the negative repelling force equals the charging force, no more electrons will move onto the plate. CHARGING A PLATE NEGATIVELY Uncharged plate offers little opposition electron movement. ©of ©o9 ©OQ 2©9 OOq ©or ©o© ©06 ©oj ©o© ©06 ©Oia CHARGE IN VOLTS Plate partially charged - slows down electron movement. Or 9= 9? Cl 0 0-0 900 eoo 900 909 909 909 909 909 909 900 909 909 009 909 ' ' 9 Plate completely charged- stops electron movement L- 3-79 CAPACITANCE IN AC CIRCUITS Capacitance (continued) Similarly, il electrons are removed from a plate by the attraction of a positive charge, the plate is positively charged. The first few electrons leave quite easily; but, as more electrons leave, a strong positive charge is built up on the plate. This positive charge attracts electrons and makes it more difficult to pull them away. When this positive attracting force equals the charging force, no more electrons leave the plate. CHARGING A PLATE POSITIVELY CHARGE IN VOLTS Plate completely charged— stops the flow of electrons from the plate. 3-80 ooooooooooooooo CAPACITANCE IN AC CIRCUITS Capacitance (continued) To see how capacitance affects the voltage in a circuit, suppose your cir- cuit contains a two plate capacitor, a knife switch and a dry cell as shown below. Assuming both plates are uncharged and the switch is open, then no current flows and t he voltage between the two plates is zero. When the switch is closed, the battery furnishes electrons to the plate con- nected to the negative terminal and takes electrons away from the plate connected to the positive terminal. The voltage between the two plates should equal the voltage between the cell terminals, or 1.5 volts. However this does not occur instantly because, in order for a voltage of 1.5 volts ’ to exist between the two plates, one plate must take excess electrons to become negatively charged, while the other must give up electrons to be- come positively charged. As electrons move onto the plate attached to the th^m l nv terr f l ^ al of th ® cell > a negative charge is built up which opposes the movement of more electrons onto the plate; and similarly, as electrons are taken away from the plate attached to the positive terminal, a positive lT ge ^ buUt . up wh \ ch °PP° ses the removal of more electrons from that jr™ Thls actl ° n on ^ he two plates is called "capacitance" and it opposes the change m voltage (from zero to 1.5 volts). It delays the change in volt- age for a limited amount of time but it does not prevent the change. 3-81 CAPACITANCE IN AC CIRCUITS Capacitance (continued) When the switch is opened the plates remain charged, since there is no path between the two plates through which they can discharge. As long as no discharge path is provided, the voltage between the plates will remain at 1.5 volts and, if the switch is again closed, there will be no effect on the circuit since the capacitor is already charged. DC CURRENT FLOW STOPS WHEN THE CAPACITOR With a DC voltage source then, current will flow in a capacitive circuit only long enough to charge the capacitor (often called a "condenser ). When the DC circuit switch is closed, an ammeter connected to read cir- cuit current would show that a very large current flows at first, since the condenser plates are uncharged. Then as the plates gain polarity and op- pose additional charge, the charging current decreases until it reaches zero — at the moment the charge on the plates equals the voltage of the DC voltage source. This current which charges a capacitor flows only for the first moment after the switch is closed. After this momentary flow the current stops, since the plates of the capacitor are separated by an insulator which does not allow electrons to pass through it. Thus capacitors or condensers will not allow DC current to flow continuously through a circuit. 3-82 CAPACITANCE IN AC CIRCUITS Capacitance (continued) While a capacitor blocks the flow of DC current it affects an AC circuit differently, allowing AC current to flow through the circuit. To see how this works, consider what happens in the DC circuit if a double-throw switch is used with the dry cell so that the charge to each plate is reversed as the switch is closed— first in one position and then in the other. When the switch is first closed the condenser charges, with each plate be- ing charged in the same polarity as that of the cell terminal to which it is connected. Whenever the switch is opened, the condenser retains the charges on its plates equal to the cell voltage. + - 3-83 CAPACITANCE IN AC CIRCUITS Capacitance (continued) If the switch is then closed in its original position no current flows, since the condenser is charged in that polarity. However, if the switch is closed in the opposite direction, the condenser plates are connected to cell termi- nals opposite in polarity to their charges. The positively charged plate then is connected to the negative cell terminal and will take electrons from the cell — first to neutralize the positive charge, then to become charged negatively until the condenser charge is in the same polarity and of equal voltage to that of the cell. The negatively charged plate gives up electrons to the cell, since it must take on a positive charge equal to that of the cell terminal to which it is connected. 3-84 CAPACITANCE IN AC CIRCUITS Capacitance (continued) If a zero-center ammeter which can read current flow in both directions is inserted in series with one of the capacitor plates, it will indicate a cur- rent flow each time the plate is charged. When the reversing switch is first closed, it shows a current flow in the direction of the original charge. Then, when the cell polarity is reversed, it shows a current flow in the op- posite direction as the plate first discharges, then charges in the opposite polarity. The meter shows that current flows only momentarily, however, each time the cell polarity is reversed. CHARGE AND DISCHARGE CURRENT OF A CAPACITOR AS THE SOURCE VOLTAGE REVERSES Suppose you switch the cell polarity fast enough so that at the instant the condenser plates become charged in one polarity, the cell polarity is re- versed. The meter needle now moves continuously — first showing a cur- rent flow in one direction, then in the opposite direction. While no elec- trons move through the air from one plate to the other, the meter shows that current is continuously flowing to and from the condenser plates. 3-85 CAPACITANCE IN AC CIRCUITS Capacitance (continued) If a source of AC voltage is used instead of the dry cell and reversing switch, the polarity of the voltage source is automatically changed each half cycle. If the frequency of the AC voltage is low enough, the ammeter will show current flow in both directions, changing each half cycle as the AC polarity reverses. The standard commercial frequency is 60 cycles so that a zero-center am- meter will not show the current flow, since the meter pointer cannot move fast enough. Even though it did, you would not be able to see the movement due to its speed. However, an AC ammeter inserted in place of the zero- center ammeter shows a continuous current flow when the AC voltage source is used, indicating that in the meter and in the circuit there is a flow of AC current. Remember that this current flow represents the con- tinuous charging and discharging of the capacitor plates, and that no actual electron movement takes place directly between the plates of the capacitor. Capacitors are considered to pass AC current, because current actually flows continuously in all parts of the circuit except the insulating material between the capacitor plates. AC CURRENT in a capacitive circuit 3-86 CAPACITANCE IN AC CIRCUITS Units of Capacitance The action of capacitance in a circuit is to store a charge and to increase its , charge if the voltage rises, or discharge if the voltage falls. Every circuit has some capacitance, with the amount depending on the ability of the circuit to store a charge. The basic unit of capacitance is the farad, but the storage capacity of a farad is much too great to use as the unit of capacitance for practical elec- trical circuits. Because of this, the units normally used are the micro- farad (equal to one-millionth of a farad) and the micromicrofarad (equal to one-millionth-millionth of a farad). Since electrical formulas use capaci- tance stated in farads, it is necessary that youare able to change the vari- ous units of capacitance to other units. Again the method of changing units is exactly like that used for changing units of voltage, current, ohms, etc. To change to larger units, the decimal point moves to the left; while to change to smaller units, the decimal is moved to the right. CHANGING UNITS OF CAPACITANCE MICROFARADS TO FARADS Move the decimal point 6 places to the left 120 itofd equals 0.000120 farad FARADS TO MICROFARAD Move the decimal point 6 places to the right 8 farads equals 8,000,000 mfd MICROMICROFARADS TO FARADS Move the decimal point 12 places to the left 1500 mmf equals 0.000000001500 farad FARADS TO MICROMICROFARADS Move the decimal point 12 places to the right 2 farads equals 2,000,000,000,000 mmf MICROMICROFARADS TO MICROFARADS Move the decimal point 6 places to the left 250 mmf equals 0.000250 mfd MICROFARADS TO MICROMICROFARADS Move the decimal point 6 places to the right 2 mfd equals 2,000,000 mmf In electrical formulas the letter C is used to denote capacitance in farads. The circuit symbols for capacitance are shown below. These symbols are also used to indicate capacitors, both fixed and variable. Most circuit capacitance is due to capacitors (condensers). CAPACITOR SYMBOLS Hh Hh Hf- Fixed Capacitors Variable Capacitors 3-87 CAPACITANCE IN AC CIRCUITS Demonstration — Current Flow in a DC Capacitive Circuit In circuits containing only capacitance, both the charge and discharge of a capacitor occur in a very short period of time. To show the circuit cur- rent flow during the charge and discharge of a capacitor, the instructor connects two 45-volt batteries in series to form a 90-volt battery. Next he connects the leads from this battery to two of the end terminals of a double-pole, double-throw switch. With the switch open, he then connects the 0-1 ma. zero-center milliammeter and the 1-mfd capacitor in series with the resistor to the switch as shown. Finally, he connects the other two end terminals together with a length of wire. The purpose of the 91, 000- ohm resistor is to limit large current surges which may damage the meter. When the instructor moves the switch to the shorted terminals, you see that there is no current flow since the capacitor is initially uncharged. Then, when he moves the switch to the battery terminals, you see that the meter pointer momentarily registers a current flow, but drops quickly to zero again as the capacitor becomes charged. 3-88 CAPACITANCE IN AC CIRCUITS Demonstration— Current Flow in a DC Capacitive Circuit (continued) If the instructor opens the switch and then moves it to the shorted ter- minals, the meter pointer indicates a momentary current flow in the op- posite direction when the switch is closed, indicating the discharge of the condenser. The instructor then charges the capacitor as before, and you notice the instantaneous current flow. He then opens the switch and returns it to its initial position. You notice no current flow, since the capacitor is already charged. When he moves the switch to the shorted terminals, the current flow in the opposite direction again shows the discharge of the condenser. 3-89 CAPACITANCE IN AC CIRCUITS Demonstration— Current Flow in a DC Capacitive Circuit (continued) Next the instructor connects the battery to the capacitor and switch in series. The capacitor is charged by closing the switch. He then opens the switch and shorts across the capacitor terminals with a screwdriver blade, making certain to hold the screwdriver only by means of the insulated handle. Notice that the capacitor discharge causes a strong arc. If you were to discharge the capacitor by touching the two terminals with your hands, the resulting electric shock— while not dangerous in itself— might cause a serious accident by making you jump. As the instructor repeatedly charges and discharges the capacitor, you see that the resulting arc is the same each time. This shows that the charge left in a capacitor when the circuit voltage is removed is always maximum in a DC circuit. CAUTION: Never discharge a capacitor while it is con- nected to the circuit voltage, whether the voltage source is a battery or AC power line. 3-90 CAPACITANCE IN AC CIRCUITS Demonstration— Current Flow in an AC Capacitive Circuit After disconnecting the DC circuit, the instructor connects the capacitor to show that AC current flows continuously in an AC capacitive circuit. One lead of the line cord is connected to a terminal of the capacitor through the switch and fuses, while the other line cord lead is connected to the remaining capacitor terminal through the 0-50 ma. AC milliam- meter . When the transformer is plugged into the AC power line outlet and the switch is closed, you see that a continuous flow of current is indi-' cated on the milliammeter. The milliammeter shows that approximately 22 ma. of AC current is flow- ing continuously. This continuous flow of circuit current is possible since the capacitor is continuously charging and discharging as the AC voltage reverses its polarity. After the instructor opens the switch, he shorts out the terminals of the capacitor with a screwdriver. Again you see that the capacitor retains a charge when the voltage is removed from the circuit. However, as the power is applied and removed several times in succession and the capacitor is discharged each time, you see that the sparks vary in size and intensity. This occurs because the amount of charge retained by the capacitor when used in an AC circuit is not always the same, since the circuit voltage may be removed while the capacitor is discharging or not yet charged. OBSERVING AC CURRENT FLOW.. To prove that capacitors appear to block DC but permit AC to pass, the instructor sets up the circuit shown to the right. When DC is applied the lamp will not glow and a DC voltmeter across the lamp will read zero volts. When AC is applied the lamp will glow and an AC voltmeter across the lamp will give a reading. 4mfd 60 W lamp 3-91 CAPACITANCE IN AC CIRCUITS Review of Capacitance in AC Circuits You have found out about capacitance and have seen how it affects the flow of current in electric circuits. Now you are ready to perform an experi- ment on capacitance to find out more about it and its effects. Before per- forming the experiment, suppose you recall what you have foune out about capacitance. CAPACITANCE — The property of a circuit which opposes any change in the circuit voltage. © “ — 0 1 Ok CAPACITOR — An electrical de- vice used to supply capacitance in a circuit. CAPACITOR CHARGE — Flow of electrons into one plate and away from the other, resulting in a neg- ative charge on one plate and a positive charge on the other. 1 mfd = 1 , 000,000 farad 1 mmf = 1 , 000 , 000 , 000,000 farad CAPACITOR DISCHARGE — Flow of electrons from tne negatively charged plate of a capacitor to the positively charged plate, elimi- nating the charges on the plate. FARAD — Basic unit of capaci- tance used in electrical formulas. PRACTICAL UNITS OF CA- PACITANCE — Microfarad (one- millionth Tarad) and micromicro- farad (one-millionth-millionth of a farad). 3-92 CAPACITORS AND CAPACITIVE REACTANCE Capacitors liasicaUy, capacitors consist of two plates which can be charged— sepa- rated by an insulating material called the "dielectric." While early con- densers were made with solid metal plates, newer types of condensers use metal foil particularly aluminum foil, for the plates. Dielectric mate- rials commonly used include a ir, mica and waxed paper. CONSIST OF leva "PCatci. AND THE 'Dielectric Plates are made of solid metal or metal foil. Dielectric materials are: air, mica and waxed paper. Three basic factors influence the capacity of a capacitor or condenser — the area of the plates, the distance between the plates (thickness of the di- electric) and the material used for the dielectric. 3-93 CAPACITORS AND CAPACITIVE REACTANCE Factors Which Affect Capacitance Plate area is a very important factor in determining the amount of capac- itance, since the capacitance varies directly with the area of the plates. A large plate area has room for more excess electrons than a small area, and thus it can hold a greater charge. Similarly, the large plate area has more electrons to give up and will hold a much larger positive charge than a small plate area. Thus an increase in plate area increases capaci- tance, and a decrease in plate area decreases capacitance. Larger plates hold more electrons 3-94 CAPACITORS AND CAPACITIVE REACTANCE Factors Which Affect Capacitance (continued) 2t^ 8 ^^o C s£?? , t£ dle f- haV 1 ° n ® aCh ° ther depends on the distance m/Sfe l 10n 0f ca P acitance depends on the two plates ui their charges, the amount of capacitance changes niltf distance between the plates changes. The capacitance between ^°t ) fi a t !f S + increases as _, the Plates are brought closer together and decreases are th to P Mrh S ^ m ?7 ed apart * This occurs because the closer the plates effect a charge on one plate wm ha " el6Ctr ° nS aPPear 8 on one plate of a condenser, electrons of®. f Vje opposite plate, inducing a positive charge on this plate. a Positively charged plate induces a negative charge on the op- P° slte Pja te> The closer the plates are to each other, the stronger the force between them, and this force increases the capacitance of a circuit. INCR easing the distance between the deaths t De&ie6tee& (tytacCfattce CAPACITORS AND CAPACITIVE REACTANCE Factors Which Affect Capacitance (continued) CHANGING THE 'Dielec&Uc MATERIAL CHANGES THE CAPACITANCE d, electric increases the capacitance. Using the same plates fixed a certain distance apart, the capacitance will change if different insulating materials are used for the dielectric. The effect of different materials is compared to that of air that is, if the con- denser has a given capacitance when air is used as the dielectric, other materials used instead of air will multiply the capacitance by a certain amount called the "dielectric constant." For example, some types of oiled paper have a dielectric constant of 3 and, if this waxed paper is placed be- tween the plates, the capacitance will be 3 times greater than air used as the dielectric. Different materials have different dielectric constants, and thus will change the capacitance when they are placed between the plates to act as the dielectric. 3-96 CAPACITORS AND CAPACITIVE REACTANCE Capacitors in Series and Parallel When y° u connect capacitors in series or in parallel, the effect on the total capacitance is opposite to that for similarly connected resistors. Connecting resistors in series increases the total resistance because it in^ henS t he res f stance through which current flows, while connect - ti?elv 6S £ SCrie ? d ® creases the total capacitance since it effec- tively mcreases the spacmg between the plates. To find the total caDaci- **** is — Seated @giia» ■■aaaaaaanBaaaij* ■> BaaBaaiar a laamMu ■■■up™ ■■mmHjmmalHiHin C ■BaaaaaaaaBBBi 1 aBBBaa^aaaa*’- The value of Z Draw the diagonal of the paral- lelogram between the inter- section of the dotted lines and the reference point. easure the length of Z on the same scale used for R and Xl to find the value of Z in ohms. In addition to showing the value of the circuit impedance, the vector solu- tion also shows the phase angle between the circuit current and voltage. The angle between the impedance vector Z and the resistance vector R is the "phase angle" of the circuit in degrees. This is the angle between the circuit voltage and current , and represents a current lag of 39 degrees. 4-3 IMPEDANCE IN AC CIRCUITS R and L Series Circuit Impedance (continued) A protractor is an angle -measuring device utilizing a semi-circular double scale marked off in degrees. To measure the phase angle of a vector relative to a reference line, the horizontal edge of the protractor is lined up with the horizontal reference vector and the protractor vertical line is lined up with the vertical vector. The degree point at which the diagonal vector intersects the semi-circular scale is the phase angle of the diagonal vector. The angle is read between zero and 90 degrees, because the phase angle will never be less than zero or greater than 90 degrees. USING A PROTRACTOR TO MEASURE PHASE ANGLE IMPEDANCE IN AC SERIES CIRCUITS R and L Series Circuit Impedance (continued) Ohm's law for AC circuits may also be used to find the impedance Z for a series circuit. In applying Ohm' s law to an AC circuit, Z is substituted for R in the formula. Thus the impedance Z is equal to the circuit voltage E divided by the circuit current I. For example, if the circuit voltage is 117 volts and the current is 0.5 ampere, the impedance Z is 234 ohms. OHM'S LAW FO AC CIRCUITS AC Jl voltage E ( V ] source 117 V W 0.5 amp Z= J In AC circuits Z= §”5 = 234 — If the impedance of a circuit is found by applying Ohm's law for AC, and the value of R is known and Xl is unknown, the phase angle and the value of Xl may be determined graphically by using vectors. If the resistance in the circuit above is known to be 200 ohms, the vector solutionis as follows: 1. Since the resistance is known to be f T 'T T TT T " T ~ ' f T T T " I I I I I M 1 1 I I I 200 ohms, the resistance vector is --------- ----ZZZZZZl^ZZZ drawn horizontally from the reference IIIIIIZII : Ii:::: - :::]: - ':: point. At the end erf the resistance vec- IIZZZIZZI I ZZZIZZZZZ l|Z I Z I tor, a dotted line is drawn perpendicu- ' lar to the resistance vector. IIjIIIZZZZIZZ I jZIZ: « ♦t'l H I M 1 H III M l +Ff 2. Using a straight edge marked to indicate the length of the impedance vector Z, find the point on the perpendicular dot- ted line which is exactly the length of the impedance vector from the refer- ence point. Draw the impedance vector between that point and the zero position. The angle between the two vectors Z and R is the circuit phase angle, and the length of the dotted line between the ends of the two vectors represents Xl- 3. Complete the parallelogram by draw- ing a horizontal dotted line between , , , , , the end of the vector Z and a vertical — line drawn up from thereference point. < tZZZZZZZZZ _j Xl is this vertical line, and its length can be read by using the same scale. In the example shown, Xl = 122 n. Phase SisssSiSsS^ls 4. Measure the phase angle with a protractor. 5. Divide R/Z = Power Factor = 200/234 = . 85 or 85%. 4-5 IMPEDANCE IN AC SERIES CIRCUITS R and L Series Circuit Impedance (continued) If the impedance and inductance are known, but the resistance is not known, both the phase angle and resistance may be found by using vectors. For example, the impedance is found to be 300 ohms by measuring the current and voltage and applying Ohm's law for AC. If the circuit induct” ance is 0.5 henry, the vector solution is as follows: 1. First the inductive reactance is computed by using the for- mula Xl = 2fZtL. If the fre- quency is 60 cycles, then Xl is 188 ohms (Xl = 6.28 x 60 x 0.5 = 188 a). Draw the vec- tor Xl vertically from the ref- erence point. At the end of this vector, draw a horizontal dotted line perpendicular to the vector Xl- 4 *L 188-0- 2. Using a straight edge marked to indicate the length of the impedance vector, find the point on the horizontal dotted line which is exactly the length of the impedance from the ref- erence point. Draw the im- pedance vector Z between that point and the reference point. The distance between the ends of the vectors Xt, and Z rep- resents the length of the re- sistance vector R. 3. Draw the vector R horizontally from the zero position and complete the parallelogram. The angle between R and Z is the phase angle of the circuit in degrees, and the length of the vector R represents the resistance in ohms. 4. Measure the phase angle with a protractor. 5. Compute the power factor. P. F. = R/Z 4-6 IMPEDANCE IN AC SERIES CIRCUITS R and L Series Circuit Impedance (continued) You have already learned how to calculate (by Ohm's Law) the Impedance of a series circuit which is composed of a coil and a resistor, and which is connected to an AC voltage source. You will now learn how to calculate the impedance of such a circuit without the use of Ohm's Law, and without making measurements. Assume, for this problem, that the inductive reactance Xl is 4 ohms and that the resistance is 3 ohms. The formula for finding the impedance in the circuit described is as follows: Because you may not be familiar with the "square root" sign (V ), and the "square" signs which are in position at the top right of the symbols for resistance and inductive reactance, these symbols or signs are ex- plained below. A simple way of showing mathematically that a number is to be multiplied once by itself, or "squared, " is to place a numeral 2 at the upper right of the number in question. So that 3 2 a 3 x 3 or 9 5^ a 5 x 5 or 25 4 2 » 4 x 4 or 16 and 12 2 » 12 x 12 or 144, etc. 4-7 IMPEDANCE IN AC SERIES CIRCUITS R and L Series Circuit Impedance (continued) The square root sign, or V indicates that you are to break down the enclosed figure to find a number which, when multiplied by itself, results in the original enclosed figure. Con sider the problem of finding the square root of 144, shown mathematically, \/l44. What number multiplied by it- self results in 144? The answer, of course, is 12. So that V 144 * 12, and/SS * 6, \l 25 * 5, and = 3, etc. The formula for impedance (Z = \l R 2 + Xl 2 ) then, indicates that you must "square" the values for R and Xl, add the two resultant figures and take the square root of the total, in order to find the impedance, Z. As given on the previous sheet, Xl is 4 ohms and R is 3 ohms. Find the impedance. R=3A By substitution Z * ^3^ + 4^ a + 16 a \/25 a 5 More often than not, the number enclosed by the square root symbol does not permit a simple answer without decimals. In that event it is necessary to find the square root which is closest to the desired numeral within the limits outlined by the instructor. For example, the instructor may elect to accept as correct an impedance value of five ohms even though the orig- inal figure required that the square root of 30 be found. (The v/30 is actually equal to 5.47.) There are, of course, methods for finding accu- rate square root answers but these need not be dealt with here. 4-8 IMPEDANCE IN AC SERIES CIRCUITS Power Factor The concept of power, particularly with regard to the calculation of power factors in AC circuits, will become a very important consideration as the circuits you work with are made more complex. In DC circuits the ex- pended power may be determined by multiplying the voltage by the current. A similar relationship exists for finding the amount of expended power in AC circuits, but certain other factors must be considered. In AC circuits the power is equal to the product of voltage and current only when the E and I are in phase. If the voltage and current waves are not in phase, the power used by the circuit will be somewhat less than the product of E and I. A review of the following principles (most of which you already know) will be helpful later on, in the discussion. 1. Power is defined as the rate of doing work. 2. A watt is the unit of electrical power. 3. Apparent power is the product of volts and amperes in an AC circuit. 4. True power is the amount of power actually consumed by the circuit. 5. True power is equal to apparent power if the voltage and current are in phase. 6. True power is equal to zero if the voltage and current are out of phase by 90 degrees. 7. True power is equal to the apparent power multiplied by a figure called the "cosine of the phase angle. " (This term will be explained shortly. ) 8. The cosine of the phase angle is called the "power factor" and is often expressed as a decimal or percentage. The power factor is important because it converts apparent power to true power. It is this true power, the kind that actually does work in the cir- cuit, which will interest you most. The cosine of the phase angle, or power factor, is explained with the help of the familiar impedance triangle diagram. The resistive component forms the base. The reactive component (X) forms the right angle with the base, and the third side called the "hypotenuse" is the resultant impedance (Z). The angle formed by the R and Z sides of the impedance triangle is called by the Greek letter 0 (theta). The angle 0 originally refers to the phase angle difference between the voltage across the resistor and the voltage across the coil in the diagram on the next sheet. IMPEDANCE IN AC SERIES CIRCUITS Power Factor (continued) Theoretically, the voltage across the coil leads the current through the circuit by a phase angle of 90 degrees. El leads the voltage across the resistor because Er is in phase with I. el In practical circuits a phase angle of 90 degrees is an impossibility be- cause every coil (and wire) has some resistance, however small. It is this resistance, and the resistance placed in the circuit by design, which reduces the phase angle to something less than 90 degrees. Consider the voltage triangle of the circuit above. Each of the voltages noted may also be expressed Ez 3 IZ E X = K E R - IR as the product of the current times the resistance, reactance, or imped- ance, as the case may be. The term, cosine of the phase angle, refers to a ratio of E R to E z and is expressed mathematically as Ed Cos e =-=r L Ez This formula is used just as well where impedance, resistance, and re- actance are the prime considerations, because ~ e R IR JL Cos ® Ez ~ IZ ” Z Cos 8 You now know all of the factors mentioned in points 7 and 8 on the previous sheet. The formula for true power should, therefore, present no difficul- ties. This formula is expressed mathematically as El Cos 8 Cos 6 P Cos 6 4-10 IMPEDANCE IN AC SERIES CIRCUITS Power Factor (continued) Suppose that you wish to find the amount of power expended (true power) in a circuit where the impedance is five ohms, the resistance is three ohms, and the inductive reactance is four ohms. The impressed voltage is ten volts AC, and the current is two amperes. X L = 40 R = 30 X L = 4 R = 3 E = 10 volts, AC 1=2 amps, AC The formula for true power is P * El Cos 6. With reference to the imped- ance triangle the Cos 0 is equal to the ratio of R divided by Z, or Cos 0 = Y" T* • 60 or 60 % Substituting in the formula for true power P = El Cos 9 P = 10 x 2 x .6 P = 12 watts of power expended The components in the circuit must be chosen of such size that they will be able to dissipate twelve watts of power at the very minimum, or burnout will occur. 4-11 IMPEDANCE IN AC SERIES CIRCUITS R and L Series Circuits Impedance Variation The impedance of a series circuit containing only resistance and induct- ance is determined by the vector addition of the resistance and the induc- tive reactance. If a given value of inductive reactance is used, the imped- ance varies as shown when the resistance value is changed. 9*Hped(i*tce. r Uanie4> WHEN Xl IS A FIXED VALUE AND R IS VARIED Impedance increases and the phase angle decreases as R increases Similarly, if a given value of resistance is used, the impedance varies as shown when the inductive reactance is changed. atfouA 9mp@dcutce Va/Uei im pedance increases and the phase angle increases as X L increases When the resistance equals zero the impedance is equal to XL, and when the inductive reactance equals zero the impedance is equal to R. While the value of Z may be determined by means of a complex mathematical formula, you will use the practical methods — a vector solution or Ohm's law for AC circuits. 4-12 IMPEDANCE IN AC SERIES CIRCUITS R and C Series Circuit Impedance If your AC series circuit consists of resistance and capacitance in series, the total opposition to current flow (impedance) is due to two factors, re- sistance and capacitive reactance. The action of capacitive reactance causes the current in a capacitive circuit to lead the voltage by 90 de- grees, so that the effect of capacitive reactance is at right angles to the effect of resistance. While the effects of both inductive and capacitive re- actance are at right angles to the effect of resistance, their effects are exactly opposite-inductive reactance causing current to lag and capaci- tive reactance causing it to lead the voltage. Thus the vector Xc, repre- senting capacitive reactance, is still drawn perpendicular to the resist- ance vector, but is drawn down rather than up from the zero position. The impedance of a series circuit containing R and C is found in the same manner as the impedance of an R and L series circuit. For example, sup- pose that in your R and C series circuit R equals 200 ohms and Xq equals 200 (dims. To find the impedance, the resistance vector R is drawn hori- zontally from a reference point. Then a vector of equal length is drawn downward from the reference point at right angles to the vector R. This vector Xc represents the capacitive reactance and is equal in length to R since both R and Xq equal 200 ohms. REPRESENTING R AND X c AS VECTORS R wm 200 A VECTORS representing resistance and capacitive reactance To complete the vector solution, the parallelogram is completed an*: a diagonal drawn from the reference point. This diagonal is the vector Z and represents the impedance in ohms (283 (dims). The angle between the vectors R and Z is the phase angle of the circuit, indicating the amount in degrees that the current leads the voltage. COMBINING VECTORS R AND X c TO FIND THE IMPEDANCE IMPEDANCE IN AC SERIES CIRCUITS R and C Series Circuit Impedance (continued) R and C series circuit impedances can be found either by using the vec- tor solution or by application of Ohm' s law. To find the impedance Z by using a vector solution, you should perform the steps outlined. 1. Compute the value of Xq by using the formula X^- = 2jttC‘ 111 * or " mula 2nris a constant equal to 6.28, f is the frequency in cycles per second and C is the capacitance in farads. 2. Draw vectors R and Xq to scale on graph paper, using a common ref- erence point for the two vectors. R is drawn horizontally to the right from the reference point and Xq is drawn downward from the refer- ence point, perpendicular to the resistance vector R. 3. Using dotted lines, a parallelogram is completed and a diagonal drawn from the reference point to the intersection of the dotted lines. The length of this diagonal represents the impedance Z as measured to the same scale as that used for R and Xq. The angle between the vectors R and Z is the phase angle between the circuit current and voltage. 4. Measure the angle in degrees with a protractor. E You can also use Ohm's law (Z = y) to find Z. After measuring the cir- cuit current and voltage, the impedance in ohms can be found by dividing the voltage by the current. For example, if the circuit voltage is 117 volts and 0.1 ampere of current flows through an AC series circuit consisting of R and C, the impedance is 1170 ohms (117 t 0.1 = 1170 ohms). 4-14 IMPEDANCE IN AC SERIES CIRCUITS R and C Series Circuit Impedance (continued) When the value of either R or Xq Is unknown, but the value of Z is known, you can find the unknown value by vector solution. If you find Z by applying Ohm's law to the AC circuit, and the value ofR is known and Xg is unknown, the first step in finding Xq is to draw the resistance vector R to scale. Next a dotted line is drawn downward at the end of and perpendicular to the vector R. A straight edge, marked to in- dicate the length of the impedance vector Z, is used to find the point on the dotted line which is exactly the length of the impedance vector Z from the reference point. Draw the vector Z between that point and the refer- ence point. Complete the parallelogram with the value of Xq being equal to the distance between the ends of the vectors R and Z. If Z and Xq are known but R is unknown, the vector Xq is first drawn to scale. A horizon- tal dotted line is drawn to the right, at the end of and perpendicular to the vector Xc. Then a straight edge is used as before to find the point on this dotted line which is exactly the length of the impedance vector Z from the reference point. Draw the vector Z between that point and the refer- ence point. Complete the parallelogram, with the value of R being equal to the distance between the ends of the vectors Xq and Z. IMPEDANCE IN AC SERIES CIRCUITS R and C Series Circuit Impedance (continued) The ratio of R to Xc determines both the amount of impedance and the phase angle in series circuits consisting only of resistance and capaci- tance. If the capacitive reactance is a fixed value and the resistance is varied, the impedance varies as shown. When the resistance is near zero, the phase angle is nearly 90 degrees and the impedance is almost entirely due to the capacitive reactance; but, when R is much greater than Xc, the phase angle approaches zero degrees and the impedance is affected more by R than Xq. WHEN X c IS A FIXED VALUE AND R IS VARIED ... Impedance increases and phase angle decreases as R increases If your circuit consists of a fixed value of resistance and the capacitance is varied, the impedance varies as shown below. As the capacitive re- actance is reduced toward zero, the phase angle approaches zero degrees and the impedance is almost entirely due to the resistance; but, as Xc is increased to a much greater value than R, the phase angle approaches 90 degrees and the impedance is affected more by Xc than R. WHEN R IS A FIXED VALUE AND X c IS VARIED 4-16 IMPEDANCE IN AC SERIES CIRCUITS L and C Series Circuit Impedance In AC series circuits consisting of inductance and capacitance, with only negligible resistance, the impedance is due to inductive and capacitive reactance only. Since inductive and capacitive reactances act in oppo- site directions, the total effect of the two is equal to their difference. For such circuits, Z can be found by subtracting the smaller value from the larger. The circuit will then act as an inductive or a capacitive reactance (depending on which is larger) having an impedance equal to Z. For ex- ample, if Xl = 500 ohms and Xq = 300 ohms, the impedance Z is 200 ohms and the circuit acts as an inductance having an inductive reactance of 200 ohms. If the Xl and Xq values were reversed, Z would still equal 200 ohms, but the circuit would act as a capacitance having a capacitive re- actance of 200 ohms. The relationships of the above examples are shown below. Z is drawn on the same axis as Xl and Xq and represents the difference in their values. The phase angle of the L and C series circuit is always 90 degrees except when Xl = Xq, but whether it is leading or lagging depends on whether Xl is greater or less than Xq. The phase angle is the angle between Z and the horizontal axis. If X L = 500 A and Xq = 300A then Z = 20011 L X L = 50011 < 4T~1 Xq = 300H < 90 c | ..U— V Z = 200A Reference point Phase angle is 90° — current lagging. Circuit acts as an inductance. If X L = 30011 and Xq = 5 00 A then Z = 2 00 A X L = 300 il< ♦ l Xq = 500A<( ^ Reference / point TT 90° Z = 2 00 A 4-17 Phase angle is 90° — current leading. Circuit acts as a capacitance. IMPEDANCE IN AC SERIES CIRCUITS R, L and C Series Circuit Impedance The Impedance of a series circuit consisting of resistance, capacitance and Inductance in series depends on three factors: R, XLandXc. If the values of all three factors are known, impedance Z maybe found as follows: Combining Hectors H Xj and Xq to find the Impedence Combine Xj^ and X^; 1. Draw vectors Xl and to scale vertically from the reference point, and subtract the smaller vector from the larger. The difference is the new vector and should be drawn to scale on the perpendicular axis as shown. Perform a vector ad- dition by subtracting a length equal to the shorter vector from the longer vector. „ — - ■ _ _ _ — — ■* : - — — — * --- - - - — 4-f -- — — -- - — — — -- x L -x c Reference point 2. Draw the vector R to scale horizon- tally, and combine it with the vector obtained in the solution of Step 1 by completing the parallelogram and drawing the diagonal. This diagonal is the vector Z, representing the circuit impedance. The angle be- tween the vectors R and Z is the circuit phase angle. ][a You can also find the impedance of the circuit by applying Ohm's law for AC circuits, after measuring the circuit current and voltage. 4-18 IMPEDANCE IN AC SERIES CIRCUITS R, L and C Series Circuit Impedance (continued) On the previous sheet you learned how to combine vectors R, Xl and Xq to find the impedance. It is a simple matter to find the phase angle be- tween the impedance and the resistance by using a protractor. Superim- pose the protractor on the vector diagram below. Take the angle reading at the point where the impedance line crosses the protractor scale. 4-19 IMPEDANCE IN AC SERIES CIRCUITS R, L and C Series Circuit Impedance (continued) The impedance of a circuit which contains R, L and C components may als o be calculated by using a variation of the impedance formula Z * \/r2 + x 2 . You have learned that it makes no difference if the reactive component, X, is inductive or capacitive in nature; the impedance is found in the same way, using the same formula for Z. Also, when both inductive and capaci- tive reactance are present in a circuit it is only necessary to subtract the smaller amount of reactance, either inductive or capacitive, as the case may be, from the larger amount and then draw in the resultant diagonal vector Z. In calculating the value for impedance in a circuit containing both inductive and reactive components use the formula Z =\/r 2 + Xe 2 where Xg is equal to Xl - Xq or vice versa, as required. In the diagram on the previous sheet, assume that Xl is seven ohms, that Xq is three ohms, and that R is three ohms. Placing these values in the impedance formula, we find Z =\/r 2 + (X L - X C ) 2 z = v/a 2 + (7 - 3) 2 = V3 2 + 4 2 Z = y/9 + 16 ** \f25~ Z = 5 ohms You will now review the method for finding the power factor and for using it to obtain true power dissipation. The apparent power is simply the volt- age times the current. When this apparent power is multiplied by the cosine of the phase angle the result is the true power value. Using an im- pedance triangle, find the power factor and true power dissipation in a cir- cuit which is composed of an equivalent reactance of four ohms, a resist- ance of three ohms, an impedance of five ohms, and which has a voltage of 2. 5 volts and a current of 500 milliamps. You have learned that the cosine of the phase angle is simply a ratio of the resistance divided by the impedance, so that Cosine of phase angle 0 = = -|- = . 60 or 60% The formula for true power is P = El Cos 0 = 2. 5 x . 5 x . 6 P = . 75 watt 4-20 IMPEDANCE IN AC SERIES CIRCUITS Demonstration — Series Circuit Impedance To see how series circuit impedance can be computed, by using vectors or by applying Ohm's law for AC to the circuit, you will participate in a demonstration of series circuit impedance. You will see how vector solu- tions are used to obtain quickly the approximate values of series cir- cuit impedance. You will see how the AC impedance is obtained by applying Ohm's law to various circuits and checking the computed impedances. Using the 5-henry inductance, the 1-mfd capacitor and the 2000-ohm re- sistor, the instructor will demonstrate the use of vectors in finding circuit impedance for the various types of AC series circuits. Although the 5- henry filter choke used for inductance actually has a DC resistance of about 60 ohms, this value is negligible in comparison to the 2000 ohm resistor and will not be considered. To find the impedance of a circuit, the induc- tive and capacitive reactances must first be computed from the known in- ductance and capacitance values. Finding Xl for a | 5- henry inductance X L = 2TTfL X L = 6.28 x 60 x 5 X L = 1884 Frequency = 60 cycles I Y 1 *C JWTC - 6.28' x - X c = 2650 Finding Xq for a 1-mfd capacitor 1 _ 1 x o.oooooi ■ o.ooom First the instructor uses the inductive and capacitive reactance formulas to obtain the values for Xl and Xq. To check your understanding of these formulas, find the inductive and capacitive reactances for inductances of 2 and 12 henries and capacitances of 2 and 5 mfd. Compare your answers with those obtained by others in the class. 4-21 IMPEDANCE IN AC SERIES CIRCUITS Demonstration— Series Circuit Impedance (continued) Using the computed values of Xl and Xq (Xl = 1884(2 and Xq = 2650(2) and the known value of R (2000 ohms), the instructor next finds the value Z for each of the various types of series circuits by means of vector solution. (For graphical purposes, values of Xl, Xq and Z are rounded off to the nearest 50 ohms to make scale drawing possible. For example, the value used for Xl is 1900 ohms and the value used for Xc is 2650 ohms.) A protractor is used to measure phase angle in degrees. 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p, is also identical to these three waveforms both in value and phase angle, since It = Ir = = Ic* 4-26 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Voltages in AC Series Circuits Just as the impedance of an AC series current cannot be found by ad ding the values of R, Xl and Xq directly, the total voltage, Ej, of an AC series circuit cannot be found by adding the individual voltages Er, El and Eq across the resistance, inductance and capacitance of the circuit. They cannot be added directly because the individual voltages across R, L and C are not in phase with each other as shown below. AC SERIES CIRCUIT VOLTAGE Er = IR The voltage Ej? across R is in phase with the cir- cuit current, since cur- rent and voltage are in phase in pure resistive circuits. The voltage El across L leads the circuit current by 90 degrees, since cur- rent lags the voltage by 90 degrees in purely Inductive circuits. Thus El crosses the zero axis going in the same direction, 90 de- grees before the current. The voltage Eg across C lags the circuit current by 90 degrees since cuirent leads the voltage by 90 de- grees in purely capacitive circuits. Thus Eg crosses the zero axis, going in the same direction, 90 de- grees after the current wave. 4-27 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Voltages In AC Series Circuits (continued) To find the total voltage in a series circuit, the instantaneous values of the individual voltages for a particular moment are added together to obtain the instantaneous values of the total voltage wavefc-m. Positive values are added directly as are negative values, and the difference between the total positive and negative values for a given moment is the instantaneous value of the total voltage waveform for that instant of time. After all pos- sible instantaneous values have been obtained, the total voltage waveform is drawn by connecting together these instantaneous values. OUT" OF" PHASE Combined instantaneous values are the result of combining the instantaneous values of Ei and E2 When combining out-of-phase voltages, the maximum value of the total voltage waveform is always less than the sum of the maximum values of- the individual voltages. Also, the phase angle (which is the angle between any two waveforms) of the total voltage wave differs from that of the indi- vidual voltages, and depends on the relative value and phase angles of the individual voltages. 4-28 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS R and L Series Circuit Voltages Suppose you consider an AC series circuit having negligible capacitance. The total circuit voltage depends on the voltage El across the circuit in- ductance and the voltage Er across the circuit resistance. El leads the circuit current by 90 degrees while Er is in phase with the circuit cur- rent; thus El leads Er by 90 degrees. To add the voltages El and Er, you can draw the two waveforms to scale and combine instantaneous values to plot the total voltage waveform. The total voltage waveform, E?, then shows both the value and phase angle of E T . The value and phase angle of E>p also may be found by drawing vectors to represent El and Er, completing the parallelogram and drawing the diagonal which represents t Ex- The angle between Er and Ex Is the phase angle between total circuit voltage, Ex> and the circuit current, Lp- Use pro- tractor to find phase angle. VECTOR ADDITION OF Fj{ AND F l { .✓Phase Angle Vi _ 4-29 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS R and C Series Circuit Voltages If your circuit consists of only R and C, the total voltage is found by com- bining Er, the voltage across the resistance, and Eq> the voltage across the capacitance. Er is in phase with the circuit current while Ec lags the circuit current by 90 degrees; thus Ec lags Er by 90 degrees. The two voltages may be combined by drawing the waveforms to scale or by us- ing vectors. AC Voltage Source Voltage vectors for a series circuit are drawn in the same manner as re- sistance, reactance and impedance vectors. Following is an example of the use of Ohm's law, as it applies to each part and to the entire series circuit. Use a protractor to find the phase angle. AND VECTOR RELATIONSHIP OF AN R AND C SERIES CIRCUIT 4-30 N|» CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS L and C Series Circuit Voltages To find the total voltage of an L and C series circuit, you need only find the difference between El and Eq since they oppose each other directly. E L leads the circuit current by 90 degrees while Eq lags it by 90 degrees. When the voltage waveforms are drawn, the total voltage is the difference between the two individual values and is in phase with the larger of the two voltages, El or Eq. For such circuits, the value of the total voltage can be found by subtracting the smaller voltage from the larger. E L ADDITION OF E L AND Ec TO FIND Ef Either or both of the voltages El and Eq may be larger than the totalcir- cuit voltage in an AC series circuit consisting only of L and C. . Greater than Ex Greater than Ex' El , Greater than Ex Individual voltages are greater than the total voltages . Greater than Ex The voltage vectors and reactance vectors for the L and C circuit are similar to each other, except for the units by which they are measured. Ohm's law applies to each part and to the total circuit as outlined below. El = Kl i- El e l x l =^ p F = — • E t E C = IXc T _ E C T. P. = P. F. x A.P. Xc Ef Xc = -^ 4-31 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS R, L and C Series Circuit Voltages To combine the three voltages of an R, L and C series circuit by means of vectors, two steps are required: O The voltages El and Ec are combined by using vectors. El ®L - ®C El - Ec Ec O The combined value of El and Er Is next combined with the i i voltage Er, using vec- tors. The result of this combination Is the total circuit voltage Et* You can use Ohm's law for any part of the circuit by substituting Xt, or Xr for R across inductors and capacitors respectively. Then, for the total circuit, Z replaces R as it is used in the original formula. 4-32 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Series Circuit Resonance In any series circuit containing both L and C, the circuit current is great- est when the inductive reactance Xl equals the capacitive reactance Xc, since under those conditions the impedance is equal to R. Whenever Xl and Xr are unequal, the impedance Z is the diagonal of a vector combina- tion of R and the difference between Xl and Xr. This diagonal is always greater than R, as shown below. When Xl and Xc are equal, Z is equal to R and is at its minimum value, allowing the greatest amount of circuit current to flow. When Xl and Xc are equal the voltages across them, El and Ec, are also equal and the circuit is said to be "at resonance." Such a circuit is called a "series resonant circuit." Both Et and E r, although equal, may be greater than E-j. which equals Er. The application of Ohm's law to the series resonant circuit is shown below. SERIES RESONANT CtocutC 4-33 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Series Circuit Resonance (continued) If either the frequency, the inductive reactance or the capacitive reactance is varied in a series circuit consisting of R, L and C,with the other values kept constant, the circuit current variation forms a curve called the "reso- nance curve." This curve shows the rise in current to a maximum value at exact resonance, and the decrease in current on either side of resonance. For example, consider a 60-cycle AC series circuit having a fixed value of inductance and a variable value of capacitance, as in a radio receiver. The circuit impedance and current variations, as the capacitance is changed, are shown below in outline form. At resonance Xl = Xc, El = Ec, Et = Er, Z is at its minimum value and It 4s at its maximum value. Resonant frequency (fr) is calculated by the formula fr ■ ^ Increasing the value of c decreases x c and varies z IMPEDANCE CURRENT RESONANCE CURVE -L Effect on current and impedance of varying the capacitance through resonance in a series circuit. Similar curves result if capacitance and frequency are held constant while the inductance is varied, and if the inductance and capacitance are held constant while the frequency is varied. 4-34 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Series Circuits Resonance (continued) The example described on the previous sheet may be discussed with the emphasis placed on a varying frequency of the input voltage and with the inductance and capacitance held constant. Variation of the voltage input frequency to an R, L, and C circuit (over a suitable range) results in an output current curve which is similar to the resonance curve on the previous sheet. When operated below the resonant frequency the current output is low and the circuit impedance is high. Above resonance the same condition occurs. At the resonant frequency the output current curve is at its peak and the impedance curve is at a minimum. The graphs of current and impedance below are typical of an R, L, C series circuit when only the input signal frequency is varied. IOO •0 40 to o CURRENT THROUGH A SERIES RESONANT CIRCUIT IS MUCH HIGHER AT THE RESONANT FREQUENCY THAN AT ANY OTHER FREQUENCY THIS GRAPH SHOWS HOW THE SERIES IMPEDANCE OF THE CIRCUIT VARIES WITH FREQUENCY FREQUENCY Further light as to the effects of reactance below and above resonance can be seen by looking at the phase angle graph below. The phase angle is clearly capacitive (negative) below resonance, and is clearly inductive (positive) above resonance. The phase angle is, of course, zero at the exact frequency of resonance. PHASE ANGLE GRAPH 4-35 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Power at Resonance in Series Circuits You have already learned that in AC circuits the power is equal to the product of the volts and amperes only when the current and voltage are in phase. If the voltage and current are notin phase the power expended will be something less than the product of E and I. The amount by which the expended power is less than the apparent power (the E times I) is deter- mined by the power factor of the circuit. This power factor is essentially the ratio of the resistance divided by the impedance of the circuit. It is often called the cosine of the phase angle (cos 0) which is formed by the R and Z in an impedance triangle. In a series circuit consisting of inductance, resistance, and capacitance the condition of resonance occurs when the inductive reactance and the ca- pacitive reactance are numerically equal and therefore completely cancel each other; see the diagram below. Frequency (Increasing — ■-) Since Xl is equal to Xc and since they oppose and therefore cancel each other out, the total impedance of the circuit (Z) must be equal to the re- sistance (R). Further, the phase angle 0 does not exist and is therefore considered to be equal to zero. If the impedance is equal to R then the power factor must be equal to 1, as follows: Z = R and „ R R , cos 0 = z = R * 1 4-36 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Demonstration — R and L Series Circuit Voltage To demonstrate the relationship of the various voltages in an R and L series circuit, the instructor connects a 1500- (dim resistor and a 5-henry filter choke to form an L and R series circuit. With the switch closed, individual voltage readings are taken across the choke and the resistor. Also, the total voltage across the series combination of the resistor and choke is measured. Notice that, if the measured voltages across the choke and resistor are added directly, the result is greater than the total volt- age measured across the two in series. The voltage El measured across the 5- henry filter choke is approximately 47.5 volts, and Eg measured across the 1500- (Am resistor is about 37.5 volts. When added directly, the voltages El and Eg total approximately 85 volte but the actual measured voltage across the resistor and filter choke in series is only about 60 volte. Using vectors to combine Ejj and E L , you see that the result is about 60 volte— the actual total circuitvolt- age as measured. 4-37 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Demonstration — R and C Series Circuit Voltage Next the instructor removes the 5-henry filter choke from the circuit, re- places it with a 1-mfd capacitor and measures the individual and total voltages of the circuit. Again you see that the sum of the voltages across the capacitor and resistor is greater than the actual measured total voltage. MtaiusUntj. the. Voltcuje. ol an R and C SERIES CIRCUIT.... The measured voltage across the capacitor is about 53 volts, while that across the resistor is approximately 30 volts. When added these voltages total 83 volts, but the actual measured voltage across the capacitor and resistor in series is only approximately 60 volts. Using vectors to com- bine the two circuit voltages, Er and Eq, you see that the result is about 60 volts, equal to the measured voltage of the circuit. Notice that, when- ever the circuit power is removed, the instructor shorts the terminals of the capacitor together to discharge it. 4-38 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Demonstration— L and C Series Circuit Voltages By replacing the 1500- ohm resistor with a 5-henry filter choke, the in- forms an L and C series circuit having negligible resistance. With the power applied, the voltages across the filter choke and capacitor are measured individually and the total voltage is measured across the series circuit. Notice that the voltage across the inductance (filter choke) alone is greater than the measured total voltage across the circuit. Add- ing the voltage across the filter choke to that across the capacitor results in a much greater value than the actual measured total circuit voltage. Using vectors to combine the two voltages, you see that the result isap- proximately equal to the measured total voltage, or about 60 volts. Al- though it is considered negligible, the resistance of the filter choke wire causes a slight difference in the computed and actual results. A 0-500 volt range AC meter is used instead of the 0-250 volt range meter, as the read- ings may exceed the 0-250 volt scale. 4-39 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Demonstration — Series Resonance To demonstrate series resonance, the instructor replaces thel-mfd capac- itor with a 0.25-mfd capacitor and inserts a 1500-ohm, 10-watt resistor in series with the 5-henry filter choke and the capacitor. This forms an R, L and C series circuit in which C will be varied to show the effect or reso- nance on circuit voltage and current. A 0-50 ma. AC milliammeter is con- nected in series with the circuit to measure the circuit current. A 0-250 volt range AC voltmeter will be used to measure circuit voltages. OBSERVING THE VOLTAGES AND CURRENT FLOW IN AN R, L w. AND C SERIES CIRCUIT C21II1J 0-250 volt range AC Voltmeter With the switch closed, you see that the current is not large enough to be read accurately since it is less than 10 ma. As the Instructor measures the various circuit voltages, you see that the voltage Er across the re- sistor is less than 10 volts, the voltage El across the filter choke is about 13 volts and the voltage Ec across the capacitor is about 55 volts. The total voltage Ex across the entire circuit is approximately 60 volts. 4-40 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Demonstration — Series Resonance (continued) By using various parallel combinations of the 0.25-mfd, 0.5-mfd, 1-mfd and 2-mfd capacitors, the instructor varies the circuit capacitance from 0.25 mfd to 3.5 mfd in steps of 0.25 mfd. Notice that he removes the cir- cuit power and discharges all capacitors used before removing or adding capacitors to the circuit. You see that as the capacitance is Increased the current rises to a maximum value reached at the point of resonance, then decreases as the capacitance Is Increased further. OBSERVING THE CIRCUIT VOLTAGES AND CURRENT CHANGE AS THE CAPACITANCE VALUE IS CHANGED "lrirb. 3j TT.J T TJT Except for the total circuit voltage ET,the measured circuit voltages vary as the capacitance Is changed. The Voltage Er across the resistor changes In the same manner as the circuit current. For capacitance values less than the resonance value, Ec Is greater than El,. Both voltages Increase as the capacitance approaches the resonance point, with El Increasing more rapidly so that at resonance El equals Eg. As the capacitance Is Increased beyond the resonance point, both Ec and El decrease In value. Ec decreases more rapidly, so that El Is greater than Ec when the cir- cuit capacitance Is greater than the value required for resonance. (The maximum El and Ec are not equal due to the relatively high resistance. If the resistance Is reduced, El and Ec will be equal at resonance.) 4-41 CURRENT, VOLTAGE AND RESONANCE IN AC SERIES CIRCUITS Review of AC Series Circuit Voltages and Current You have found that the rules for AC series circuit voltage and current are the same as those for DC circuits, except that the various circuit volt- ages must be added by means of vectors because of the phase difference between the individual voltages. Now review what you have found out about AC series circuit current, voltages and resonance, and how Ohm's law applies to an AC series circuit. AC SERIES CIRCUIT CURRENT — The cur- rent is the same in all parts of a series circuit. AC SERIES CIRCUIT VOLTAGES — E R is in phase with the current, El leads the current by 90 degrees, and Eq lags the cur- rent by 90 degrees. -►0 999 -vwwv- - / uuu v - Hh SERIES CIRCUIT RESONANCE — At reso- nance Xl = Xc, El = Ec, current is maxi- mum and Z = R. P. F. = 100%. 4-42 ALTERNATING CURRENT PARALLEL CIRCUITS AC Parallel Circuit Combinations Electrical equipment is usually con- nected in parallel across AC power lines, forming parallel combinations of R, L and C. As in series circuits, every parallel circuit contains a cer- tain amount of resistance, inductive reactance and capacitive reactance; but for a given circuit any of these factors may be so negligible that it can be disregarded. The same combinations of R, L and C which are used to form the various types of series circuits may also be used to form parallel circuits. If one factor is negligible, the three possible combinations are R and L, R and C, or L and C, while a fourth type of par- allel circuit contains R, L and C. You have found out how R, L andC, individually and in various series circuit combinations, affect AC cur- rent flow, voltage, phase angle and power. Now you will find out how cur rent, voltage, phase angle and pow- er are affected by the various paral- lel combinations. c 4-48 ALTERNATING CURRENT PARALLEL CIRCUITS Voltages in AC Parallel Circuits You will remember that in a parallel DC circuit the voltage across each of the parallel branches is equal. This is also true of AC parallel circuits, the voltages across each parallel branch are equal and also equal E«r, the total voltage of the parallel circuit. Not only are the voltages equal, but they are also in phase. For example, if the various types of electrical equipment shown below— a lamp (resistance), a filter choke (inductance) and a capac itor (capacitance ) —are connected in parallel, the voltage across each is exactly the same. AC PARALLEL CIRCUIT BRANCH VOLTAGES ARE EQUAL AND IN PHASE 117 volts | AC Power Line u 117 voMs 117 volts 117 volts Regardless of the number of parallel branches, the value of the voltage across them is equal and in phase. All of the connections to one side of a parallel combination are considered to be one electrical point, as long as the resistance of the connecting wire may be neglected. 4-44 ALTERNATING CURRENT PARALLEL CIRCUITS Currents in AC Parallel Circuits The current flow through each individual branch is determined by the op- position offered by that branch. If your circuit consists of three branches — one a resistor, another an inductor and the third a capacitor— the cur- rent through each branch depends on the resistance or reactance of that branch. The resistor branch current Id is in phase with the circuit voltage Ep, while the inductor branch current It. lags the circuit voltage by 90 de- grees and the capacitor branch current Iq leads the voltage by 90 degrees. Because of the phase difference between the branch currents of an AC par- allel circuit, the total current Ip cannot be found by adding the various branch currents directly— as it can for a DC parallel circuit. When the waveforms for the various circuit currents are drawn in relation to the common circuit voltage waveform, Xl and Xq again are seen to cancel each other since the waveforms for Ip, and 1^ are exactly opposite in po- larity at all points. The resistance branch current Ir, however, is 90 de- grees out of phase with both II and 1^ and, to determine the total current flow by using vectors, Ir must be combined with the difference between II and Iq. ALTERNATING CURRENT PARALLEL CIRCUITS Currents in AC Parallel Circuits (continued) To add the branch currents in an AC parallel circuit, the instantaneous values of current are combined, as voltages are in a series circuit, to ob- tain the instantaneous values of the total current waveform. After all the possible instantaneous values of current are obtained, the total current waveform is drawn by connecting together the instantaneous values. The maximum value of It is less than the sum of the maximum values of the individual currents, and is out of phase with the various branch cur- rents. With respect to the circuit voltage, the total current either leads or lags Ic and II between zero and 90 degrees, depending on whether the inductive or capacitive reactance is greater. A graph showing the various circuit currents and the circuit voltage of an AC parallel circuit is similar to the graph of circuit current and voltages for an AC series circuit. They differ in that the different series circuit voltages are drawn with reference to total circuit current, while for par- allel circuits the different currents are drawn with reference to the total circuit voltage. 4-46 ALTERNATING CURRENT PARALLEL CIRCUITS R and L Parallel Circuit Currents If your AC parallel circuit consists of a resistance and inductance con- nected in parallel, and the circuit capacitance is negligible, the total cir- cuit current is a combination of Ir (the current through the resistance) and *L fth e current through the inductance). Ir is in phase with the circuit voltage E-p while Ip, lags the voltage by 90 degrees. To find the total current Ip, you can draw Ir and II to scale and in the proper phase relationship to each other and combine the corresponding ^ instantaneous values to plot the total t current waveform. This waveform then shows both the maximum value and the phase angle of If. VECTOR ADDITION OF Ir AND I L COMBINING WAVEFORMS I R AND I L You can also use an easier method to find the value and phase angle of Ip. By drawing vectors to scale representing Ir and Ip,, then combining the vectors by completing the parallelogram and drawing the diagonal, you can obtain both the value and phase angle of I-p. The length of the diagonal represents the value of It, while the angle between It and Ir is the phase angle between total circuit voltage, Et, and the total circuit current, bp. 4-47 ALTERNATING CURRENT PARALLEL CIRCUITS R and C Parallel Circuit Currents The total current of an AC parallel circuit which consists only of R and C is found by combining In (the resistance current) and Ic (the capacitance current). I R isin phase with the circuit voltage E-r, while Ic leads the volt- age by 90 degrees. To find the total current and its phase angle when Ir and l£ are known, you ca*. draw the waveforms of Ir and Ic or their vectors. FINDING THE TOTAL CURRENT IN AN R AND Q CIRCUIT. . . Combining Ir and Ic Waveforms Adding Ir and Ic Vectors While the capacitance does Increase the circuit current, only the resistance current consumes power, so that parallel circuits containing a capacitance branch will pass more current than is necessary to provide a given amount of power. This means that the power line wires which carry current to such circuits must be larger than if the circuit were purely resistive. 4-48 ALTERNATING CURRENT PARALLEL CIRCUITS L and C Parallel Circuit Currents When your parallel circuit consists only of L and C, the total current is equal to the difference between 1^ and Iq since they are exactly opposite in phase relationship. When the waveforms for I l and Iq are drawn, you see that all the instantaneous values of It and Iq are of opposite polarity. If all the corresponding combined instantaneous values are plotted to form the waveform of *T> the maximum value of this waveform is equal to the difference between II and Iq. For such circuits the total current can be found by subtracting the smaller current, II or Iq, from the larger. FINDING T1IF TOTAL CIIRKKNT IN AN 1. AND C I ’A KALI FI * HOT! 1 The relationships and paths of circuit currents for L and C circuits are shown below. Line current lT s ^L“k 10 Diagram of circuit showing circulating current ihs parallel circuit can also be considered as consisting of an Internal and external circuit. Since the current flowing through the Inductance Is ex- actly opposite In polarity to that which is flowing through the capacitance at the same time, an Internal circuit Is formed. The amount of current flow around this Internal circuit is equal to the smaller of the two currents, I L and Iq. The amount of current flowing through the external circuit (the voltage source) Is equal to the difference between I L and Iq. 4-49 ALTERNATING CURRENT PARALLEL CIRCUITS L and C Parallel Circuit Currents (continued) The relationship between the various currents in a parallel circuit con- sisting of L and C is illustrated in the following example. A capacitor and an inductor are connected in parallel across a 60- cycle, 150- volt source, so that Xl = 50 ohms and Xq = 75 ohms. The currents in the circuit are: lL = _E x L i E 150 - Xc " 75 " 4A - Since II and Iq are exactly opposite in phase, they have a canceling effect on each other. Therefore, the total current I* = II = 3 - 2 = 1A. Due to the phase relationship of II and Iq, the current flow through the capacitor is always opposite in direction to the current flow through the inductor. Using this phase relationship and the Kirchhoff's law relating to currents approaching and leaving a point in a circuit, you can see in the diagram that Ic and It are approaching point A while II is leaving point A. For this particular circuit, II must be equal to the sum of It and Iq. Il - h + *C 3=1 + 2 Since I^is made up partially of Iq, it can be seen that Iq must flow through the inductor. Therefore, Ic flows through the capacitor and through the inductor and then back through the capacitor. The result of the opposing phase of II and Ic is to form an internal circuit, whose circulating cur- rent has a value equal to the smaller of II and Ic, in this case Ic* If the values of Xl and Xc were reversed, II would be the circulating cur- rent. The smaller current (II or Iq) is always the circulating current. 4-50 ALTERNATING CURRENT PARALLEL CIRCUITS R, L and C Parallel Circuit Currents To combine the three branch currents of an R, L and C alternating-current parallel circuit by means of vectors requires two steps as outlined below: 1 . The currents I L and Iq are com- bined by using vectors. (Both the value, which may be obtained by direct subtraction, and the phase angle of this combined current are required.) The combined value of It, and Iq is then combined with % to. ob- tain the total current. © fl'c-'i In an R, L and C circuit — as in the L and C circuit — a circulating current equal to the smaller of the two currents Ij_, and Iq flows through an inter- nal circuit consisting of the inductance branch and the capacitance branch. The total current which flows through the external circuit (the voltage source) is the combination of Ir and the difference between the currents II and Iq. 4-51 ALTERNATING CURRENT PARALLEL CIRCUITS AC Parallel Circuit Impedance The impedance of a parallel circuit can be found using complicated vector or mathematical solutions, but the most practical method is to apply Ohm's law for AC to the total circuit. Using Ohm's law for AC, the impedance Z for all AC parallel circuits is found by dividing the circuit voltage by the total current To find the impedance of a parallel circuit the total current is first found by using vectors; then Ohm's law for AC is applied to find Z. The steps used to find Z for the various types of AC parallel circuits are outlined below. Vectors fo £iad PARALLEL CIRCUIT IMPEDANCE AND POWFR FACTOR R and L R and C L and C R, L and C Parallel Circuits Parallel Circuits Parallel Circuits Parallel Circuits DRAW BRANCH CURRENTS TO SCALE TO FIN'D I T AND USE PROTRACTOR TO DETERMINE PHASE ANGLE POWER FACTOR = 7— ! t 4-52 ALTERNATING CURRENT PARALLEL CIRCUITS Demonstration— R and L Parallel Circuit Current and Impedance The current flow and the practical method of obtaining the impedance of an R and L parallel circuit is demonstrated first. The instructor connects a 2500-ohm, 20-watt resistor and a 5-henry filter choke in parallel across the AC power line through a step-down autotransformer, to form an AC parallel circuit of R and L. A 0-50 ma. AC milliammeter is connected to measure the total circuit current and a 0-250 volt range AC voltmeter is used to measure circuit voltage. With the power applied to the circuit, you see that the circuit voltage is about 60 volts and the total circuit current is approximately 40 milliamperes. To measure the individual currents Ir and II through the resistor and the filter choke, the instructor first connects the milliammeter to measure only the resistor current, then to measure only the filter choke current. You see that the milliammeter reading for IrIs about 24 ma.,and the cur- rent indicated for II is approximately 32 ma. The sum of these two branch currents Ir and II then is 56 ma. while the actual measured total circuit current is about 40 ma., showing that the branch currents must be added by means of vectors. The calculated value of the impedance for this R and L circuit is 1500 ohms (60 +■ 0.040 = 1500), indicating that the parallel connection of R and L reduces circuit impedance. The total circuit impedance is less than that of either branch of the circuit, since R = 2500IL and Xl = 188411. 4-53 ALTERNATING CURRENT PARALLEL CIRCUITS Demonstration— R and C Parallel Circuit Current and Impedance Next the instructor replaces the 5-henry filter choke with a 1-mfd capaci- tor and repeats the previous demonstration. The total circuit voltage and current (E-p and It) Is measured, and then the branch currents Ir and Ic are measured through the resistor and capacitor. You see that the total circuit current It is approximately 32 ma., while the measured branch currents Ir and Ic are about 24 ma. arid 23 ma. re- spectively. Again you see that the total current is less than the sum of the branch currents, due to the phase difference between Ir and Iq. The total impedance is about 1875 ohms (60 -r 0.032 » 1875), a value less than the opposition offered by either branch alone, since R = 2000 -fi- and Xq = 2650 iL. _ TOT^rctftClJtt 4-54 ALTERNATING CURRENT PARALLEL CIRCUITS Demonstration — L and C Parallel Circuit Current and Impedance To demonstrate the opposite effects of L and C in a parallel circuit, the 2500-ohm resistor is replaced by the 5-henry filter choke forming an L and C parallel circuit. Again the instructor repeats each step of the demonstration, first measuring the total circuit current, then that of each branch. You see that the total circuit current is about 9 ma., while II is about 32 ma. and 1^ is about 23 ma. Thus, the total current is not only less than that of either branch but is actually the difference between the currents II and Iq. The total circuit impedance of the L and C circuit is 6700 ohms (60 0.009 e 6700), a value greater than the opposition of either the L or C branch of the circuit. Notice that when L and C are both present in a parallel circuit the impedance increases, which is opposite in effect to that of a series circuit where combining L and C results in a lower impedance. 4-55 ALTERNATING CURRENT PARALLEL CIRCUITS Demonstration— R, L and C Parallel Circuit Current and Impedance By connecting a 2500-ohm, 20-watt resistor In parallel with the 5-henry filter choke and the 1-mfd capacitor, the Instructor forms an R, L and C parallel circuit. To check the various currents and find the total circuit Impedance he measures the total circuit current, then the individual cur- rents through the resistor, filter choke and capacitor in turn. Observe that the total circuit current increases and that the individual cur- rents are the same as those previously measured in each branch. You see that Ir is 24 ma., Il is 32 ma. and IC is 23 ma. Again you see that the sum of the Individual currents is much greater than the actual measured total current of 29 ma. The total circuit impedance is about 2070 ohms (60 + 0.029 ■= 2070). The total circuit current is the sum of the resistor current Ir and the combined inductance and capacitance currents II and Ic, added by means of vectors. 4-56 ALTERNATING CURRENT PARALLEL CIRCUITS Review of AC Parallel Circuit Current and Impedance Consider what you have found out so far about AC parallel circuits. While reviewing parallel circuit current and Impedance, compare the effects of series and parallel connections of R, L and C in AC circuits. © iVi-'c © Ci-'ci/fxj AC PARALLEL CIRCUIT IMP EDANCE AND POWER FACTOR — The impedance of an AC parallel circuit id equal to the circuit voltage divided by the total circuit current. The power factor equals the resistive current di- vided by the total circuit current. 4-57 RESONANCE IN AC PARALLEL CIRCUITS Parallel Circuit Resonance In a parallel circuit containing equal Xl and Xr, the external circuit cur- rent is equal to that flowing through the parallel resistance. If the circuit contains no parallel resistance, the external current is zero. However, within a theoretical circuit consisting only of L and C and Xl = Xc,a large current called the "circulating current" flows, using no current from the power line. This occurs because the corresponding instantaneous values of the currents II and Ic always flow in opposite directions and, if these values are equal, no external circuit current will flow. This is called a "parallel resonant" circuit. Since Ic = lL,the line current is zero and the circulating current is maximum. Because no external current flows in a resonant parallel circuit consist- ing only of L and C, the impedance at resonance is infinite, II equals Ic, and the total circuit current It is zero. Since these effects are exactly opposite those of series resonance, parallel resonance is sometimes called "anti-resonance." Ohm's law for AC when applied to a parallel resonant circuit can be used to determine the value of the internal circu- lating current. COMPUTING THE C 1 RC U LA T i N U > C U RRE N T OF A PARALLEL RESONANT CIRCUIT RESONANCE IN AC PARALLEL CIRCUITS Parallel Circuit Resonance (continued) As in the case of a series resonant circuit, if either the frequency, in- ductive reactance or capacitive reactance of a circuit is varied and the two other values kept constant, the circuit current variation forms a resonance curve. However, the parallel resonance curve is the opposite of a series resonance curve. The series resonance current curve increases to a maximum at resonance then decreases as resonance is passed, while the parallel resonance current curve decreases to a minimum at resonance RESONANCE IN AC PARALLEL CIRCUITS Demonstration — Parallel Circuit Resonance To show how parallel resonance affects parallel circuit current, the in- structor connects a 0.25-mfd capacitor and a 5-henry filter choke in par- allel to form an L and C parallel circuit. A 0-50 ma. AC milliammeter and a 0-250 volt range AC voltmeter are connected to measure circuit cur- rent and voltage. This circuit is connected to the AC power line through a switch, fuses and step-down autotransformer. When the switch is closed, you observe that the circuit current is about 30 ma. and the voltage is ap- proximately 60 volts. The total current indicated by the meter reading is actually the difference between the currents II and Ic through the inductive and capacitive branches of the parallel circuit. Because a meter connected in series with either of the branches would add resistance to that branch, causing inaccurate readings, the branch currents are not read. The circuit volt- age remains constant In parallel circuits so that, if a fixed value of induct- ance is used as One branch of the circuit, the current in that branch re- mains constant. If the capacitance of the other branch is varied, its cur- rent varies as the capacity varies, being low for small capacitance values and high for large capacitance values. The total circuit current is the difference between the two branch currents and is zero when the two branch currents become equal. As the instructor increases the circuit capaci- tance, the total current will drop as the Current Ic increases toward the constant value of 1^, will be zero when Ic equals II and then will rise as IC becomes greater than II. 4-60 RESONANCE IN AC PARALLEL CIRCUITS Demonstration— Parallel Circuit Resonance (continued) ™ r * es * he ®£ rcuit capacitance in steps of 0.25 mfd from 0.25 mfd through 2.5 mfd. Observe that the current decreases from ap- ma - to a minimum value less than 10 ma., then rises toa value beyond the range of the milliammeter. The current at resonance 2*5®. *f aCh ^® r ° be ? a . use the circuit branches are not purely capacitive ^lL n ^ C V V I and 9 annot J be so in a practical parallel circuit. You also ob- S2T L t I^{iif e l VOl ^ ge d ? es not change across either the branches or the total parallel circuit as the capacitance value is changed. THE EFFECT OF VARYING CAPACITANCE VALUES ON TOTAL CIRCUIT CURRENT Current After the value of capacitance has been varied through the complete ranee of values, the value which indicates resonance— minimum current flow— !. s - ed to s i?? w , that the circulating current exceeds the line current at « Th «! mstructor again measures the line current of the parallel ♦h ir f U i t,t «® n ?°nnects the milliammeter to measure only the cur- th t .^uctive branch. You see that the total current is less than 10 ma., yet the circulating current is approximately 30 ma. CHECKING THE VALUE OF CIRCULATING CURRENT IN A PARALLEL RESONANT CIRCUIT lx is less than 10 ma. Circulating current equals II and is greater than I T 4-61 RESONANCE IN AC PARALLEL CIRCUITS Review of Parallel Circuit Resonance You have found that the effect of parallel circuit resonance on circuit cur- rent is exactly opposite to that of series resonance. Also, you have seen parallel resonance demonstrated, showing how it affects both line current and circulating current. Before performing the experiment on parallel resonance, suppose you review its effects on current and voltage. PARALLEL RESONANCE LINE CURRENT _ The line current is minimum in a parallel resonant circuit. Ifiiwi* PARALLEL RESONANC E CIRCULATING CuilENT — The circulating current is maximum in a parallel resonant clr( ^^^ ? ” "H- AC Voltage Source i & parallel RESONANC E VOLTAGE — The voltage of parallel circuit branches at res- onance is the same as the voltage when the circuit is not at resonance. 4-62 ALTERNATING CURRENT SERIES- PARALLEL CIRCUITS Complex AC Circuits Many AC circuits are neither series nor parallel circuits but are a com- bination of these two basic circuits. Such circuits are called "series- parallel ' or "complex" circuits and, as in DC circuits, they contain both parallel parts and series parts. The values and phase relationships of the voltages and currents for each particular part of a complex circuit de- pend on whether the part is series or parallel. Any number of series- parallel combinations form complex circuits and, regardless of the circuit variations, the step-by-step vector solution is similar to the solution of DC complex circuits. The parts of the circuit are first considered sepa- ra e y, then the results are combined. For example, suppose your circuit consists of the series- parallel combination shown below, with two sepa- rate series circuits connected in parallel across a 120-volt AC power me. The vector solution used to find the total circuit current, total im - pedance and the circuit phase angle is outlined below: SERIES PARALLEL CIRCUIT branclf fs found* 1 br ? nch currents II and I 2 , the impedance of each d se P arat ® 1 y b y usin S vectors. The current values are then determined by applying Ohm's law to the branches separately. FINDING THE IMPEDANCES OF EACH BRANCH ALTERNATING CURRENT SERIES- PARALLEL CIRCUITS Complex AC Circuits (continued) Although you know the branch currents Ii and , the total current It c ^' not befound by adding Ii and Io directly. Since they are out of phase, the instantaneous values for each branch current are not equal. To find the phase relationship between Ii and 12 so that they may be added bv using vectors, the voltage and current vectors for each series branch must first be drawn separately. (Since the values of Ii and I2 areknown, the voltages across the various parts of each series branch can be found by applying Ohm’s law.) jTAGE and current vectors for each branch Eri ■ IjR| * 0.24 x 300 * 12V Eli • liX L ■ 0.24 x 400 » 9«v E L aMV i /E T «120V *1 is »« I / Phase with £^ Eri „72I e R2 * *2*4 “ 0 23 * 500 " 115V E C » IjXc * 0.23 x 150 * 34.5V 1 2 is in phase I, . 0.23A '*“ h Eh 2 V... — ---?*»• Et* Ec ■ 34.5V 120V The vector solutions for each separate branch, when drawn to scale, show both the values and the phase relationships between the branch currents and the total circuit voltage, Et* To show the phase relation between Ii and Io, they are redrawn with respect to Et which is drawn horizontally as the reference vector. Draw U down in relation to Ej at the angle found vectorally. Ii lags the voltage as the branch has the inductor in it. Draw 12 up in relation to Ex.at the angle found vectorally. 12 ieadstac voltage as the branch has the capacitor in it. Lay the protractor at the end of II with the vertical line up and the base horizontal to the reference line ET- Mark off an angle equal to the angle of the I2 vector. Next lay the pro- tractor at the end of I2 with the vertical line down and the base horizontal to the reference line Et- Mark off an angle equal to the angle of the Ii vector. Complete the parallelogram by drawing a dotted line from the end of each vector to the point marked off from your protractor. From the reference point draw a line to where the two dotted lines cross. This vec- tor represents the total current of the circuit. Mteasure with the protractor the angle between It and Et and this will be the phase angle of the circuit. Protractor Base ••• ,v.v WWW AVJ & >X; 1 B ■-M |l p *»,• SijjS iP msm *&*&%% || mmsm ALTERNATING CURRENT SERIES- PARALLEL CIRCUITS Complex AC Circuits (continued) Series-parallel circuits may be even more complex than the one just il- lustrated* For example, suppose that the series- parallel circuit is con- nected in series with an inductance and a resistance as shown below. A COMPLEX CIRCUIT CONTAINING A SERIES -PARALLEL CIRCUIT To find the total circuit current and impedance, the vectors It and Ex are redrawn with Ij as the horizontal reference vector. The phase angle be- tween these two vectors is positive, indicating that inductance more than cancels capacitance in the parallel part of the circuit. Therefore, this part of the circuit can be replaced by an R and L series circuit since the value of E t in the parallel circuit is the result of adding the vector volt- ages, Ejj and El. By completing the parallelogram, the voltages El and E r across this series circuit can be determined. The resistance and in- ductive reactance (Xl g and Kg) of this equivalent series circuit can be found by using the vector voltages and the total current, then apply ing Ohm’s law. (Although the voltage across the parallel part of the complex circuit may not be 120 volts, the computed values of R and Xn for the equivalent circuit are the same regardless of the voltage, actual or as- sumed, across this part of the circuit.) FINDING THE EQUIVALENT SERIES CIRCUIT OF THE SERIES -PARALLEL CIRCUIT (T^Draw Ex in relation to lx Find individual voltages which combine to form Ex © Since It flows _ through L e and R e * x L fi 09 = ( 96A ^ Ex jl20V Er 110V R - 110 Vor Equivalent Series Circuit X Le - 96 JT R e = 282 JT 4-65 ALTERNATING CURRENT SERIES-PARALLEL CIRCUITS Complex AC Circuits (continued) By replacing the parallel part of the complex circuit with the equivalent R e and Xt, c values, the circuit becomes a series circuit. Combining the two values of R and the two values of Xj_, results in a simple R and L series circuit, which would have the same effect on total circuit current and voltage as the entire complex circuit. To find the value of total cir- cuit impedance and current flow, the simple series circuit is then solved by using vectors. Substituting the EQUIVALENT SERIES CIRCUIT for the SERIES-PARALLEL CIRCUIT L Ri l — 1 WWW Ra L a I WAAAAr — f 500A 300A | C Hh *2 -VWAVv — 1 Ra -WWW V- La -'IRRP- R a = 500 A X La = 300A Le -'0WT- R e WWWW- X Le = 96A Re = 282A L Ri i \QStfb WWW | W' R a L a r#wwM w — k I 500A 300 A C lb r 2 -WWW \r ;0 T» eS this Total R 500 282 782 A Total X L 300 + 96 396 A Total Series Circuit Equivalent f Rt -wwwv- XLt - W i 782A 396 A 4-66 ALTERNATING CURRENT SERIES -PARALLEL CIRCUITS Demonstration of Complex Circuits For demonstration purposes, a series-parallel circuit containing resist- ance, inductance and capacitance is used. The instructor will demonstrate the method of solving such circuits to find the total circuit current, the branch currents, the impedance and the equivalent series circuit. The calculated results will be checked with actual voltage and current meas- urements, and you will see how the values compare. Because pure induct- ances and pure capacitances are only theoretical, there will be a noticeable difference between the actual and calculated results. However, the meas- ured results will show that the calculations are accurate enough for prac- tical use in electrical circuits. The instructor connects a 500-ohm resistor, a 1000-ohm resistor, a 1-mfd capacitor and a 5-henry filter choke to form the complex circuit shown below. Because the filter choke has a DC resistance of approximately 50 ohms, the total resistance of the Rand L branch of the circuit isl050ohms. R2 is rated as a 1050-ohm resistor rather than a 1000-ohm resistor. 4-67 ALTERNATING CURRENT SERIES-PARALLEL CIRCUITS Demonstration— Vector Solution of Complex Circuits Before applying power to the circuit, the instructor demonstrates the vec- tor solution to find the total current, branch currents and impedance. First the values of Xl and Xq are computed, using 60 cycles as the power line frequency. Rounded off to the nearest 50 ohms, these values are 1900 ohms for Xj_, and 2650 ohms for X(j. Using the known values of Rj an<^R 2 together with the computed values of Xt and Xr . the impedances of each series branch are found separately by Y i i 1 3 — - — . noil n nmionA Wrtlfo ffO is mg vectoi using vectors. From these values of impedance and a source voltage of 60 volts, the values of the branch currents Ij and I 2 are found. FIND THE BRANCH CURRENTS Ii AND I2 Branch 1 4 -ww — Ui-oog x i - - *L 1900 A 60 *1 = Zl = 270D 22 ma. = 28 ma. R2 1050a Next the individual voltages of each branch are drawn with respect to their corresponding branch current, to find the relationship between Ex and each of the branch currents individually. Ij Ij is in phase with Erj 22 ma. : iiXc = — 28 ma. 12 is in phase with Er 2 These individual relationships are drawn with reference to the common total voltage vector, Ex, and the two branch current vectors are combined to find the total circuit current. From the computed value of the total cur- ... . . m 11 A A 1J i.1 i.1.1 vm ERl = ll^l E C = IlXc 4. Draw the branch current vectors with respect to a common total circuit volt- age vector, then combine the currents to find the total circuit current. Cal- culate the total impedance of the circuit. Z T=| 4-70 TRANSFORMERS The Importance Of Transformers When you studied AC circuits, you learned that alternating current as a source of power has certain advantage over direct current. The most im- portant advantage of AC is that the voltage level can be raised or lowered by means of a transformer. You remember it is better to transmit power over long distances at a high voltage and low current level, since the IR drop due to the resistance of the transmission lines is greatly reduced. To transmit AC power at a high voltage low current level, the generated voltage is fed into a transformer. The transformer raises the voltage, and since power depends on both voltage and current, a higher voltage means the same amount of power will require a lower current. At the load end of the transmission line, another transformer reduces the voltage to the level necessary to operate the load equipment. For example, at Niagara Falls AC is generated at 6000 volts, stepped up by transformers to 120,000 volts and distributed over long transmission lines, stepped down at different points to 6000 volts for local distribution, and finally stepped down to 220 and 110 volts AC for lighting and local power use. Transformers are used in all types of electronic equipment, to raise and lower AC voltages. It is important for you to become familiar with trans- formers, how they work, how they are connected into circuits, and pre- cautions in using them. TRANSFORMERS How a Transformer Works When AC flows through a coil, an alternating magnetic field is generated around the coil. This alternating magnetic field expands outward from the center of the coil and collapses into the coil as the AC through the coil varies from zero to a maximum and back to zero again. Since the alter- nating magnetic field must cut through the turns of the coil an emf of self induction is induced in the coil which opposes the change in current flow EMF OF SELF INDUCTION Field expansion U Field contraction M AC current flow O Opposition to current flow offered by counter -emf TRANSFORMERS How a Transformer Works (continued) A simple transformer consists of two coils very close together, electri- cally insulated from each other. The coil to which the AC is applied is called the "primary. " It generates a magnetic field which cuts through the turns of the other coil, called the "secondary," and generates a voltage in it. The coils are not physically connected to each other. They are, how- ever, magnetically coupled to each other. Thus, a transformer transfers electrical power from one coil to another by means of an alternating mag- netic field. Assuming that all the magnetic lines of force from the primary cut through all the turns of the secondary, the voltage induced in the secondary will de- pend on the ratio of the number of turns in the secondary to the number of turns in the primary. For example, if there are 1000 turns in the sec- ondary and only 100 turns in the primary, the voltage induced in the sec- ondary will be 10 times the voltage applied to the primary = 10). Since there are more turns in the secondary than there are in the primary, the transformer is called a "step-up transformer." If, on the other hand, the secondary has 10 turns and the primary has 100 turns, the voltage induced in the secondary will be one-tenth of the voltage applied to the primary ^100 = fo^ ® nce there are less turns in the secondary than there are in the primary, the transformer is called a "step-down transformer." Trans- formers are rated in KVA because it is independent of power factor. 4-73 TRANSFORMERS How a Transformer Works (continued) A transformer does not generate electrical power. It simply transfers electric power from one coil to another by magnetic induction. Although transformers are not 100 percent efficient, they are very nearly so. For practical purposes, their efficiency is considered to be 100 percent. Therefore, a transformer can be defined as a device that transfers power from its primary circuit to the secondary circuit without any loss (as- suming 100 percent efficiency). Since power equals voltage times current, if Eplp represents the primary power and E S I S represents the secondary power, then Eplp = E s Is- If the primary and secondary voltages are equal, the primary and secondary cur- rents must also be equal. Suppose Ep is twice as large as E s . Then, in order for E D I D toequal E s I s ,I p must be one half of I s . Thus a transformer which steps voltage down, steps current up. Similarly, if Ep is only half as large as E s , I p must be twice as large as I s and a transformer which steps voltage up, steps current down. Transformers are classified step-down or step-up only in relation to their effect on voltage. as TRANSFORMERS Transformer Construction Transformers designed to operate on low frequencies have their coils, called "windings," wound on iron cores. Since iron offers little resistance to magnetic lines, nearly all the magnetic field of the primary flows through the iron core and cuts the secondary. The iron core increases the efficiency of the transformer to 98 or 99 percent, which can practically be considered 100 percent, or "no loss." Iron cores are constructed in three main types — the open core, the closed core and the shell type. The open core is the least expensive to build— the primary and the secondary are wound on one cylindrical core. The magnetic path is partly through the core, partly through the air. The air path opposes the magnetic field, so that the magnetic interaction or "link- age" is weakened. The open core transformer is inefficient and never used for power transfer. Th? closed core improves the transformer efficiency by offering more iron paths and less air path for the magnetic field, thus increasing the magnetic "linkage" or "coupling." The shell type core further increases the mag- netic coupling and therefore the transformer efficiency, because it pro- vides two parallel magnetic paths for the magnetic field. Thus maximum coupling is attained between the primary and secondary. TRANSFORMER CORE CONSTRUCTION . . . . 4-75 TRANSFORMERS Transformer Losses Not all of the electrical energy from the primary coil is transferred to the secondary coil. A transformer has some losses; and the actual efficien- cy, although usually greater than 90 percent, is less than 100 percent. Transformer losses are generally of two types — "copper losses" and "core losses." Copper losses represent the power loss in resistance of the wire in the windings. These are called copper losses since copper wire usually is used for the windings. Although normally the resistance of a winding is not high, current flow through the wire causes it to heat, using power. This power can be computed from the formula I 2 R, where R is the coil wire resistance and I is the current through the coil. results in COPPER LOSSES in a transformer Core losses are due to eddy currents and hysteresis. The magnetic field which induces current in the secondary coil also cuts through the core material, causing a current, called "eddy current," to flow through the core. This eddy current heats the core material— an indication that power is being used. If the resistance of the eddy current path is increased less current will flow, reducing the power losses. By laminating the core ma- terial, that is by using thin sheets of metal insulated by varnish — the cross- section of each current path is diminished, and the resistance to eddy cur- rent flow increases. LAMINATED CORE CONSTRUCTION reduces Eddy Current and Power Loss Hysteresis loss depends on the core material used. Each time the AC cur- rent reverses in the primary winding, the field in the core reverses its magnetic polarity. This field reversal requires a certain amount of power, resulting in a loss called "hysteresis loss." Some materials, such as silicon steel, change polarity easily, and when these materials are used as the core material, hysteresis loss is reduced to a minimum. 4-76 TRANSFORMERS The Power Supply Transformer Transformers are designed for many different uses and frequencies. The type of transformer you will probably be most concerned with is the power supply transformer. It is used to change the 117 volt 60 cycle power fre- quency to whatever 60 cycle voltage is needed to operate motors, lighting circuits and electronic equipment. The illustration shows a typical power supply transformer for electronic equipment. You see that the secondary consists of three separate windings — each secondary winding supplying a different circuit with its required voltage. The multiple secondary eliminates the need for three separate transformers, saving cost, space and weight. The iron core is shown by the standard symbol. Each secondary has three connections. The middle connection is called the "center tap," and the voltage between the center tap and either outside connection is one half the total voltage across the winding. Iron Core Black PRIMARY Black Red § s o o a g Red-Yellow Oj» " g Red High- -Voltage W inding Yellow fc> Yellow-Blue [ 5 - volt IP * Yellow [Winding Green Green-Yellow ^ 6 -volt Green _[ Winding — o J SECONDARY WINDINGS The number of turns shown in a schematic does not necessarily indicate whether the transformer is step-up or step-down. The windings are color- coded by the manufacturer, to indicate the separate secondary windings, their use and the way they are to be connected. The color-code shown is a standard one, but manufacturers may use other color-codes or may use numbers to indicate the proper connections. 4-77 TRANSFORMERS Other Types of Transformers In addition to power transformers, which operate at 60 cps (cycle per sec- ond), there are transformers designed to operate at different frequencies. The audio transformer is designed to operate on the range of "audio" fre- quencies — those frequencies which are audible to the human ear— -20 cps to 20,000 cps. The audio transformer has an iron core and is similar in appearance to the power transformer. In receivers and transmitters, frequencies much higher than the audio range are used, and these frequencies are called "radio frequencies" — 100,000 cps or 100 kilocycles (kc) and higher. Radio frequency (rf) trans- formers do not have iron cores because at such frequencies the core losses would be too great, and therefore rf transformers are "air core" trans- formers. The coils are wound on a non-magnetic form. The illustrations show a receiver rf transformer and a transmitter rf transformer. In the transmitter rf transformer, the windings are spaced far apart because of the high voltages used. All these transformers have only one primary winding and are called "single phase" transformers. Other transformers, which operate from three AC voltages, are called "three phase" transformers. These trans- formers will be discussed in the section on alternators. 4-78 TRANSFORMERS Autotransformers The autotransformer differs from other transformers in that it has only one winding, rather than two or more as in ordinary transformers. Part of this winding is used for both primary and secondary, while the rest of the winding acts as either the primary or secondary exclusively, depend- ing on whether the autotransformer is used to step down or step up the voltage. A step-up autotransformer uses a portion of the total winding as the pri- mary. AC current flow in this portion of the winding causes an expanding and collapsing field, which cuts across all of the coil turns and induces a higher voltage across the entire coil than that across the portion used as the primary. The end terminals of the coil then can be used as a secondary winding having a higher voltage than that of the primary section. If the entire coil is used as the primary winding and only a portion is used for the secondary, the secondary voltage is less than that of the primary. When so connected, the autotransformer is used to step down voltage. In the autotransformer, part of the winding is common to both the primary and secondary and carries both currents. Autotransformers require less wire since only one coil is used and they are less expensive than two-coil transformers. However, autotransformers do not isolate the primary and secondary circuits and cannot be used in many electrical and electronic circuits for this reason. 4-79 TRANSFORMERS Troubleshooting Since transformers are an essential part of the equipment you will work with, you should know how to test and locate troubles that develop in trans- formers. The three things that cause transformer failures are open wind- ings, shorted windings, and grounds. When one of the windings in a transformer develops an "open," no current can flow and the transformer will not deliver any output. The symptom of an open-circuited transformer is that the circuits which derive power from the transformer are dead. A check with an AC voltmeter across the trans- former output terminals will show a reading of zero volts. A voltmeter check across the transformer input terminals shows that voltage is pre- sent. Since there is voltage at the input and no voltage at the output, you conclude that one of the windings is open. Next you check the transformer windings for continuity. After disconnecting all of the primary and sec- ondary leads, each winding is checked for continuity, as indicated by a re- sistance reading taken with an ohmmeter. Continuity, (a continuous circuit) is indicated by a fairly low resistance reading, while the open winding will indicate an infinite resistance on the ohmmeter. In the majority of cases the transformer will have to be replaced, unless of course the break is accessible and can be repaired. DETECTING AND FINDING AN OPEN CIRCUIT CONTINUITY CHECK I i .1 1 r .!> 1 1 mi ! Reads winding resistance £ 4-80 TRANSFORMERS Troubleshooting (continued) When a few turns of a secondary winding are shorted, the output voltage drops. The symptoms are that the transformer overheats due to the large circulating currents- flowing in the shorted turns and the transformer out- put voltage is lower than it should be. The winding with the short gives a lower reading on the ohmmeter than normal. If the winding happens to be a low voltage winding, its normal resistance reading is so low that a few shorted turns cannot be detected by using an ordinary ohmmeter. In this case, a sure way to tell if the transformer is bad is to replace it with another transformer. If the replacement transformer operates satisfac- torily it should be used and the original transformer repaired or dis- carded, depending upon its size and type. TRANSFORMERS Troubleshooting (continued) Sometimes a winding has a complete short across it. Again, one of the symptoms is excessive overheating of the transformer due to the very large circulating current. The heat often melts the wax inside the trans- former, which you can detect quickly by the odor. Also, there will be no voltage output across the shorted winding and the circuit across the wind- ing will be dead. In equipment which is fused, the heavy current flow will blow the fuse before the transformer is damaged completely. If the fuse does not blow, ; the ! shorted winding may burn out. The short may be in the external circuit connected to the winding or in the winding itself. The way to isolate the short is to disconnect the external circuit from the winding. If the voltdge is normal with the external circuit disconnected the short is in the external circuit. If the voltage across the winding is still zero, it means the short is in the transformer and it will have to be replaced. DETECTING AND FINDING A COMPLETE SHORT •il-HHUi IN A TRANSFORM HR WINDING HSBij if Excessive overheating >» s f , t n i • K 1^1 Con 1/ i S1 Complete Short To circuit Melting 1 f Wax x I Find the shorted w inding with the voltmeter. Zero reading Voltmeter iii ! St! D i s e on ne c t transformer load to see if sh< < rt is in external circuit Zero reading U o replacement transformer. piiiiiiiii! Normal reading 4-82 TRANSFORMERS Troubleshooting (continued) Sometimes the insulation at some point in the winding breaks and the wire becomes exposed. If the bare wire is at the outside of the winding, it may touch the inside of the transformer case, shorting the wire to the case and grounding the winding. If a winding develops a ground, and a point in the external circuit con- nected to this winding is also grounded, part of the winding will be shorted out. The symptoms will be the same as those described for a shorted winding and the transformer will have to be replaced. You can check for a transformer ground by connecting the megger between one side of the wind' ing in question and the transformer case, after all the transformer leads have been disconnected from the circuit. A zero or low reading on the megger shows that the winding is grounded. 4-83 TRANSFORMERS Demonstration— Voltage Measurements To demonstrate power transformer action in stepping voltage up or down, the instructor uses a power transformer made up of one primary winding, and three secondary windings consisting of a step-up high voltage winding and two step-down low voltage windings, all center-tapped. See the sche- matic below. The instructor carefully separates the winding leads to make certain none are touching each other or the case. Then he attaches a line cord to the primary leads. A 0-1000 volt range AC voltmeter is set up to take meas- urements. He plugs the line cord into the 110-volt socket and measures the AC voltage across the different transformer windings. With the volt- meter leads placed across the primary, the voltmeter reads 110 volts, the line voltage. Next, he places the voltmeter leads across the two out- side high voltage leads. These leads can be identified by their red color. Notice that the voltmeter reads a very high voltage. Knowing the high vol- tage and the primary voltage, you can easily determine the turns ratio be- tween the secondary high voltage winding and the primary by dividing the secondary voltage by the primary voltage. Now the instructor measures the voltage between the high voltage center tap and first one and then the other high voltage lead. Notice that the vol- tage is exactly half that across the outside terminals. He repeats the above voltage measurements with the two low voltage step-down windings. 4-84 TRANSFORMERS Demonstration— Resistance Measurements Next the instructor measures the resistance of the various windings of the power transformer with an ohmmeter. (Caution: This reading must be taken with no power applied to the transformer, otherwise the ohmmeter would be damaged.) The ohmmeter leads are first placed across the pri- mary winding, then across the entire high voltage secondary. Observe that the resistance of the high voltage winding is much higher than the resist- ance of the primary. This is because the high voltage winding has many more turns than the primary winding. As the instructor measures the re- sistance from the center tap of the high voltage winding to either end you see that resistance equals one-half that of the full winding. Next the ohmmeter leads are placed across the two low voltage windings Observe that the ohmmeter reads practically zero ohms for both windings. This is because these windings have few turns of comparatively large di- ameter wire. You could not tell if these windings were shorted by means of a resistance measurement. The only way to check the low voltage wind- ings is to measure their output voltage with all circuits disconnected from them. If the voltage readings are normal, the windings are probably in good condition. 4-85 ALTERNATING CURRENT CIRCUITS Review of AC Circuits You have learned about AC circuits, and have investigated the various factors which affect AC current flow. This review sums up all these factors, and gives you the opportunity to check yourself on each point. Lst Half Cycle 2nd Hall Cycle ALTERNATING CURRENT — Current flow which reverses its direction at regular Intervals and is constantly changing in magnitude. SINE WAVE — A continuous curve of all the instantaneous values of an AC current or volt- age. INDUCTANCE — The property of a circuit which opposes any change in the current flow. INDUCTIVE REACTANCE — The action of inductance in opposing the flow of AC current and in causing the current to lag the voltage. CAPACITANCE — The property of a circuit which opposes any change in the circuit voltage. CAPACITIVE REACTANCE — The action of capacitance in op- posing the flow of AC current and in causing the current to lead the voltage. Power Cujr«nt Voltage POWER — In AC circuits true power is equal to El x cos 6, where the power factor (cos 6) is equal to the ratio of the resistance divided D by the impedance (%). 4-86 ALTERNATING CURRENT CIRCUITS Review of AC Circuits (continued) i T = Ir = II = ic VWW-'WHRT |h R l c AC. SERIES CIRCUIT — The current is the same in all parts of the circuit, while the voltage divides across the circuit and differs in phased SERIES RESONANCE — When Xl and Xc are equal, a series circuit is at resonance having minimum impedance and maxi- mum current. II L R — WWW— *2 j i i— 'COT'-WWMr -www — 'OtJtr — 4 c r 3 • — II — VWWWr- 1 AC PARALLEL CIRCUIT — The voltage is the same across each parallel branch while the current divides to flowthrough the various branches, with the branch currents differing in phase and amplitude. PARALLEL RESONANCE — When Xl and Xq are equal, a parallel circuit is at resonance, having maximum impedance and minimum line current. At reso- nance, the circulating current is greater than the line current. AC SERIES- PARALLE L CIRCUIT — The current divides to flow through the parallel branches while the voltage divides across series parts of the circuit. Volt- age and current phase relation- ships for each part of the circuit depend on whether the part is made up of resistance, inductance or capacitance. 4-87 TRANSFORMERS Review Here is a review of the most important points about the transformer. TRANSFORMER ACTION— The method of transferring electrical energy from one coil to another by means of an alternating magnetic field. The coils are not physically connected. They are only magneti- cally coupled. The alternating mag- netic field generated in one coil cuts through the turns of another coil and generates a voltage in that coil. PRIMARY AND SECONDARY WIND - INGS — The coil which generates the alternating magnetic field is called the "primary." The coil inwhicha voltage is induced by the alternating magnetic field is called the "sec- ondary." The voltage induced in the secondary depends upon the turns ratio between the secondary and primary. STEP-UP AND STEP-DOWN TRANSFORMER— If there are more turns in the secondary than in the primary, the transformer is "step- up," and the secondary voltage will be higher than the primary voltage. If there are fewer turns in the sec- ondary than in the primary, the transformer is "step-down," and the secondary voltage will be lower than the primary voltage. The ratio of the secondary voltage to the primary voltage is equal to the turns ratio. PRIMARY AND SECONDARY CUR- RENT — The power delivered by a transformer is equal to the power put into the transformer, assuming 100 percent efficiency. Stating this in terms of a formula, Eplp = E s Is- From this formula it can De seen that, if the transformer steps up the voltage, it will reduce the current. In other words, the transformer changes the current in the opposite direction to the change in voltage. 4-88 TRANSFORMERS Review (continued) TRANSFORMER LOSSES; Transformers designed for low frequencies are wound around iron cores to offer a low resistance path for the magnetic lines of force. This allows for a maximum amount of coupling between the primary magnetic field and the secondary winding, with the result that en- ergy is transferred to the secondary at low loss. Transformers do suffer losses which are of three types: (1) the i2r loss incurred by current flow- ing through the resistance of the windings, (2) eddy current losses caused by induction currents in the core material and (3) hysteresis losses caused by the reversal of core polarity each time the magnetic field reverses. o e AUTOTRANSFORMBRS: In addition to the two windingtransformers, there are single winding transformers called "autotransformers." Electrical energy is transferred from one part of the coil to another part of the coil by magnetic induction. The voltage and currents vary in the same manner as in the two winding transformers. STEP-DOWN STEP-UP TRANSFORMER TROUBLES: 1. One of the windings can develop an open circuit. 2. Part or all of one winding can become shorted. 3. A ground can develop. In troubleshooting a transformer, a voltmeter and ohmmeter are used to locate the trouble. If a transformer is defective, it usually must be replaced. OPEN PARTIAL COMPLETE GROUNDED 4-89 INDEX TO VOL. 4 (Note: A cumulative index covering all five volumes in this series will be found at the end of Volume 5.) AC parallel circuit, 4-43 currents in, 4-45, 4-46 impedance of, 4-52 voltages in, 4-44 Autotransformers, 4-79 Complex AC circuits, 4-63 to 4-66 Current flow, in AC series circuit, 4-26 Currents, in AC parallel circuits, 4-45, 4-46 in L and C parallel circuit, 4-49, 4-50 in R and C parallel circuit, 4-48 in R and L parallel circuit, 4-47 in R, L and C parallel circuit, 4-51 Demonstration, Complex Circuits, 4-67 to 4-69 L and C Parallel Circuit Current and Impedance, 4-55 L and C Series Circuit Voltages, 4-39 Ohm's Law for AC Circuits, 4-23, 4-24 Parallel Circuit Resonance, 4-60, 4-61 Resistance Measurements of Transformer, 4-85 R and C Parallel Circuit Current and Impedance, 4-54 R and C Series Circuit Voltage, 4-38 R and L Parallel Circuit Current and Impedance, 4-53 R and L Series Circuit Voltage, 4-37 R, L and C Parallel Circuit Current and Impedance, 4-56 Series Circuit Impedance, 4-21, 4-22 Series Resonance, 4-40, 4-41 Voltage Measurements of Transformer, 4-84 Impedance, AC parallel circuit, 4-52 AC series circuit, 4-1 to 4-25 L and C series circuit, 4-17 R and C series circuit, 4-13 to 4-16 R and L series circuit, 4-2 to 4-8 R, L and C series circuit, 4-1 8 to 4-20 variation of, 4-12, 4-16 impedance triangle, 4-7, 4-8 L and C parallel circuit currents, 4-49, 4-50 L and C series circuit, 4-17 voltages in, 4-31 Ohm's law, for AC circuits, 4-5 Parallel circuit resonance, 4-58, 4-59 Power factor, in series AC circuits, 4-9 to 4-1 .1 Resonance, parallel circuit, 4-58, 4-59 series circuit, 4-33 to 4-36 Review, AC Circuits, 4-86, 4-87 AC Complex Circuits, 4-70 AC Parallel Circuit Current and Impedance, 4-57 AC Series Circuit Voltages and Current, 4-42 Parallel Circuit Resonance, 4-62 Series Circuit Impedance, 4-25 Transformers, 4-88, 4-89 R and C circuit, voltages in, 4-30 R and C parallel circuit currents, 4-48 R and C series circuit, 4-13 to 4-16 R and L parallel circuit currents, 4-47 R and L series circuit, 4-2 to 4-8 voltages in, 4-29 R, L and C parallel circuit currents, 4-51 R, L and C series circuit, 4-18 to 4-20 voltages in, 4-32 Series circuit, resonance in, 4-33 to 4-36 Series-parallel AC circuits, 4-63 to 4-66 Transformers, 4-71 to 4-74 construction of, 4-75 losses in, 4-76 troubleshooting, 4-80 to 4-83 types of, 4-77 to 4-79 Voltages, in AC parallel circuits, 4-44 in AC series circuits, 4-27, 4-28 in L and C series circuit, 4-31 in R and C series circuit, 4-30 in R and L series circuit, 4-29 in R, L and C series circuit, 4-32 4-91 HOW THIS OUTSTANDING COURSE WAS DEVELOPED: In the Spring of 1951, the Chief of Naval Personnel, seeking a streamlined, more efficient method of presenting Basic Electricity and Basic Electronics to the thousands of students in Navy specialty schools, called on the graphio- logical engineering firm of Van Valkenburgh, Nooger & Neville, Inc., to prepare such a course. This organization, specialists in the production of complete “packaged training programs,” had broad experience serving in- dustrial organizations requiring mass-training techniques. These were the aims of the proposed project, which came to be known as the Common-Core program: to make Basic Electricity and Basic Electronics completely understandable to every Navy student, regardless of previous education; to enable the Navy to turn out trained technicians at a faster rate (cutting the cost of training as well as the time required), without sacrificing subject matter. The firm met with electronics experts, educators, officers-in -charge of various Navy schools and, with the Chief of Naval Personnel, created a dynamic new training course . . . completely up-to-date . . . with heavy emphasis on the visual approach. First established in selected Navy schools in April, 1953, the training course comprising Basic Electricity and Basic Electronics was such a tremendous success that it is now the backbone of the Navy’s current electricity and electronics training program!* The course presents one fundamental topic at a time, taken up in the order of need, rendered absolutely understandable, and hammered home by the use of clear, cartoon-type illustrations. These illustrations are the most effec- tive ever presented. Every page has at least one such illustration — every page covers one complete idea! An imaginary instructor stands figuratively at the reader’s elbow, doing demonstrations that make it easier to understand each subject presented in the course. Now, for the first time, Basic Electricity and Basic Electronics have been released by the Navy for civilian use. While the course was originally de- signed for the Navy, the concepts are so broad, the presentation so clear — without reference to specific Navy equipment — that it is ideal for use by schools, industrial training programs, or home study. There is no finer training material! *“ Basic Electronics.” the second portion of this course, is available as a separate series of volumes. JOHN F. RIDER PUBLISHER, INC, 116 WEST 14th ST., N. Y. 11, N. Y. No. 169-5 basic electricity by VAN VALKENBIIRGH, NOOGER & NEVILLE, INC. VOL. 5 DC GENERATORS & MOTORS ALTERNATORS & AC MOTORS POWER CONTROL DEVICES a RIDER publication $ 2.25 basic electricity by VAN VALKENBURGH, NOOGER & NEVILLE, INC. VOL. 5 JOHN F. RIDER PUBLISHER, INC. 116 W. 14th Street • New York 11, N. Y. First Edition Copyright 1954 by VAN VALKENBURGH, NOOGER AND NEVILLE, INC. All Rights Reserved under International and Pan American Conventions. This book or parts thereof may not be reproduced in any form or in any language without permission of the copyright owner. Library of Coiigress Catalog Card No. 54-12946 Printed in the United States of America PREFACE The texts of the entire Basic Electricity and Basic Electronics courses, as currently taught at Navy specialty schools, have now been released by the Navy for civilian use. This educational program has been an unqualified success. Since April, 1953, when it was first installed, over 25,000 Navy trainees have benefited by this instruc- tion and the results have been outstanding. The unique simplification of an ordinarily complex subject, the exceptional clarity of illustrations and text, and the plan of pre- senting one basic concept at a time, without involving complicated mathematics, all combine in making this course a better and quicker way to teach arid learn basic electricity and electronics. The Basic Electronics portion of this course will be available as a separate series of volumes. In releasing this material to the general public, the Navy hopes to provide the means for creating a nation-wide pool of pre-trained technicians, upon whom the Armed Forces could call in time of national emergency, without the need for precious weeks and months of schooling. Perhaps of greater importance is the Navy’s hope that through the release of this course, a direct contribution will be made toward increasing the technical knowledge of men and women throughout the country, as a step in making and keeping America strong. Van Valkenburgh , Nooger and Neville , Inc. New York , N. Y. October, 1954 iii TABLE OF CONTENTS Vol. 5 — Basic Electricity Elementary Generators 5-1 Direct Current Generators 5-21 Direct Current Motors 5-44 DC Motor Starters 5-64 DC Machinery Maintenance and Troubleshooting 5-68 Alternators 5-82 Alternating Current Motors 5-92 Power Control Devices 5-113 iv ELEMENTARY GENERATORS The Importance of Generators You are alliamiliar with flashlights, portable radios and car lighting sys- tems— all of which use batteries as their source of power, hi these appli- cations the current drawn from the battery is comparatively small and, therefore, a battery can supply the current for a long period of time, even without recharging. Batteries work very nicely when they supply devices which require very little current. Many kinds of electrical equipment require large amounts of current at a high voltage in order to do their job. For example, electric lights and heavy motors require larger voltages and currents than those furnished by any practical sized battery. As a result, sources of power other than bat- teries are required to supply large amounts of power. These large sources of power are supplied by rotating electrical machines called "generators. " Generators can supply either DC or AC power. In either case, the gener- ator can be designed to supply very small amounts of power or else it can be designed to supply many hundreds of kilowatts of power. 5-1 ELEMENTARY GENERATORS The Importance of Generators (continued) The world as we know it would be practically at a standstill without the electrical energy supplied by generators. Look about you and you will see proof of electrically generated energy in action. Our modern lighting systems, our factories — in fact, our entire industrial life is directly or indirectly energized by the electrical power output from rotating electrical generators. A large city soon would become a "ghost town" if its generators were put out of action. The electrical generator is as im portant to our modern way of living as the action of the heart is to the maintenance of life in your own body. ELEMENTARY GENERATORS Review of Electricity from Magnetism You will recall that electricity can be generated by moving a wire through a magnetic field. As long as there is relative motion between the conductor and the magnetic field, electricity is generated. If there is no relative mo- tion between the conductor and the magnetic field, electricity is not gener- ated. The generated electricity is actually a voltage, called an "induced voltage, " and the method of generating this voltage by cutting a magnetic field with a conductor is called "induction. " You also know that this in- duced voltage will cause a current to flow if the ends of the conductor are connected through a closed circuit- -in this case, the meter. 5-3 ELEMENTARY GENERATORS Review of Electricity from Magnetism (continued) You know that the amount of voltage induced in the wire cutting through the magnetic field depends upon a number of factors. First, if the speed of the relative cutting action between the conductor and the magnetic field in- creases, the induced emf increases. Second, if the strength of the magnetic field increases, the induced emf increases. Third, if the number of turns cutting through the magnetic field is increased, the induced emf is again in- creased. The polarity of this induced emf will be in such a direction that the result- ant current flow will build up a field to react with the field of the magnet, and oppose the movement of the coil. This phenomenon illustrates a prin- ciple known as "Lenz's Law" which states that in all cases of electromag- netic induction, the direction of the induced emf is such that the magnetic field it sets up tends to stop the motion which produces the emf. FACTORS WHICH DE TERMINE INDUCED EMF STRENGTH o . . . THE SPEED OF CONDUCTOR THROUGH MAGNETIC FIELD 5-4 ELEMENTARY GENERATORS Review of Electricity from Magnetism (continued) You also know that the direction of the generated current flow is determined by the direction of the relative motion between the magnetic field and the cutting conductor. If the relative motion is toward each other, the current flows in one direction; and, if the relative motion is away from each other, the current flows in the opposite direction. DIRECTION OF RELATIVE MOTION DETERMINES DIRECTION OF CURRENT FLOW To sum up what you already know about electricity from magnetism: 1) moving a conductor through a magnetic field generates an emf which pro- duces a current flow; 2) the faster the conductor cuts through the field, the more turns there are and the stronger the magnetic field — the greater the induced emf and the greater the current flow; and 3) reversing the direc- tion of movement of the conductor reverses the polarity of the induced emf and, therefore, reverses the direction of current flow. 5-5 ELEMENTARY GENERATORS Practical Generators You already know that you can generate electricity by having a conductor cut through a magnetic field. This is essentially the principle of operation of any generator from the smallest to the giants which produce kilowatts of power. Therefore, in order to understand the operation of practical gen- erators, you could examine an elementary generator, made of a conductor and a magnetic field, and see how it can produce electricity in usable form. Once you know how a basic generator works, you will have no difficulty in seeing how the basic generator is built up into a practical generator. PRINCIPLE AS THE . . . . PRACTICAL GENERATOR 5-6 ELEMENTARY GENERATORS Elementary Generator Construction An elementary generator consists of a loop of wire placed so that it can be rotated in a stationary magnetic field to cause an induced current in the loop. Sliding contacts are used to connect the loop to an external circuit in order to use the induced emf. The pole pieces are the north and south poles of the magnet which supplies the magnetic field. The loop of wire which rotates through the field is called the "armature." The ends of the armature loop are connected to rings called "slip rings," which rotate with the armature. Brushes ride up against the slip rings to pick up the electricity generated in the armature and carry it to the external circuit. the ELEMENTARY GENERATOR Load In the description of the generator action as outlined on the following sheets, visualize the loop rotating through the magnetic field. As the sides of the loop cut through the magnetic field, they generate an induced emf which causes a current to flow through the loop, slip rings, brushes, zero-center current meter and load resistor — all connected in series. The induced emf that is generated in the loop, and therefore the current that flows, depends upon the position of the loop in relation to the magnetic field. Now you are going to analyze the action of the loop as it rotates through the field. 5-7 ELEMENTARY GENERATORS Elementary Generator Operation Here is the way the elementary generator works. Assume that the arma- ture loop is rotating in a clockwise direction, and that its initial position (• u \ In P° sition A > the loop is perpendicular to tl^e mag- n f, , * *i. d and the black and white conductors of the loop are moving par- r ii *■-* j he ma S netic field. If a conductor is moving parallel to a magnetic field, it does not cut through any lines of force and no emf can be generated in the conductor. This applies to the conductors of the loop at the instant they go through position A— no emf is induced in them and, therefore, no current flows through the circuit. The current meter registers zero. As the loop rotates from position A to position B, the conductors are cutting through more and more lines of force until at 90 degrees (position B) they are cutting through a maximum number of lines of force. In other words, between zero and 90 degrees, the induced emf in the conductors builds up from zero to a maximum value. Observe that from zero to 90 degrees the black conductor cuts down through the field while at the same time the white conductor cuts up through the field. The induced emfs in both conductors are therefore in series -adding, and the resultant voltage across the brushes (the terminal voltage) is the sum of the two induced emfs, or double that of one conductor since the induced voltages are equal to each other. The cur- rent through the circuit will vary just as the induced emf varies— being zero at zero degrees and rising up to a maximum at 90 degrees. The cur- rent meter deflects increasingly to the right between positions A and B, in- dicating that the current through the load is flowing in the direction shown. The direction of current flow and polarity of the induced emf depend on the direction of the magnetic field and the direction of rotation of the armature loop. The waveform shows how the terminal voltage of the elementary gen- erator varies from position A to position B. The simple generator drawing on the right is shown shifted in position to illustrate the relationship be- tween the loop position and the generated waveform. 5-8 ELEMENTARY GENERATORS Elementary Generator Operation (continued) As the loop continues rotating from position B (90 degrees) to position C (180 degrees), the conductors which are cutting through a maximum num- ber of lines of force at position B cut through fewer lines, until at po- sition C they are moving parallel to the magnetic field and no longer cut through any lines of force. The induced emf therefore will decrease from 90 to 180 degrees in the same manner as it increased from zero to 90 degrees. The current flow will similarly follow the voltage variations. The generator action at positions B and C is illustrated. ELEMENTARY GENERATORS Elementary Generator Operation (continued) From zero to 180 degrees the conductors of the loop have been moving in the same direction through the magnetic field and, therefore, the polarity of the induced emf has remained the same. As the loop starts rotating be- yond 180 degrees back to position A, the direction of the cutting action of the conductors through the magnetic field reverses. Now the black con- ductor cuts up through the field, and the white conductor cuts down through the field. As a result, the polarity of the induced emf and the current flow will reverse. From positions C through D back to position A, the current flow will be in the opposite direction than from positions A through C. The generator terminal voltage will be the same as it was from A to C except for its reversed polarity. The voltage output waveform for the complete revolution of the loop is as shown. 5-10 ELEMENTARY GENERATORS Left-Hand Rule You have seen how an emf is generated in the coil of the elementary gen- erator. There is a simple method for remembering the direction of the emf induced in a conductor moving through a magnetic field; it is called the "left-hand rule for generators." This rule states that if you hold the thumb, first and middle fingers of the left hand at right angles to one another with the first finger pointing in the flux direction, and the thumb pointing in the direction of motion of the conductor, the middle finger will point in the di- rection of the induced emf. "Direction of induced emf" means the direc- tion in which current will flow as a result of this induced emf. You can re- state the last part of the left-hand rule by saying the tip and base of the middle finger correspond to the minus and plus terminals, respectively, of the induced emf. THE GENERATOR HAND RULE DIRECTION OF EJd.F. 5-11 ELEMENTARY GENERATORS Elementary Generator Output Suppose you take a closer look at the output waveform of the elementary generator and study it for a moment. DC voltage can be represented as a straight line whose distance above the zero reference line depends upon its value. The diagram shows the DC voltage next to the voltage waveform put out by the elementary AC gen- erator. You see the generated waveform does not remain constant in value and direction, as does the DC curve. In fact, the generated curve varies continuously in value and is as much negative as it is positive. The generated voltage is therefore not DC voltage, since a DC voltage is defined as a voltage which maintains the same polarity output at all times. The generated voltage is called an "alternating voltage," since it alternates periodically from plus to minus. It is commonly referred to as an AC voltage— 'the same type of voltage that you get from the AC wall socket. The current that flows* since it varies as the voltage varies, must also be alternating. The current is also referred to as AC current. AC current is always associated with AC voltage — an AC voltage will always cause an AC current to flow. A B C D A 5-12 ELEMENTARY GENERATORS Converting AC to DC by Use of the Reversing Switch seen , how y°“ r elementary generator has generated AC. Now you r.Lr5^? r ¥S,l^ r A S«SI* 0r C1 ” m0dltled 10 PU * ° UlDC At these points, the conductors of the loop reverse their direction through the SrfrHnn * ield ‘ Y ° u the Parity of the induced emf depends^n reverses * the ^°" ductor ™ ove s through a magnetic field. If the direction everses, the polarity of the induced emf reverses. Since the Iood con- ha"ve S rot f. tlng |- hrou S h the field , the conductors of the loop will always Snhr n hi rn ff g induced in them. Therefore, the only way that DC One wav to rfn e t d h/ r ° m t th K e generator is to convert the generated AC to DC Dut 1S haVG a SWltch hooked U P across the generator out- thp'o.uit ^! t h Can b ! so conn ected that it will reverse the polarity of thP P t ^tage every time the polarity of the induced emf changes inside e o5X2, SWitCh is illustrated in the diagram. The switch must this ^ rfono “ anuai ;y every time the polarity of the voltage changes. If lain,! d ^ n ®’ the volt age applied to the load will always have the same do- althi y ,Jh nd f th ® cu f rent ii° w through the resistor will not reverse direction although it will rise and fall in value as the loop rotates. CHANGING AC TO DC USING A--- REVERSING SWITCH REVERSING SWITCH ELEMENTARY GENERATORS Converting AC to DC by Use of the Reversing Switch (continued) Consider the action of the switch as it converts the generated AC in vary- ing DC across the load resistor. The first illustration shows the load re sistor the switch the generator brushes, and the connecting wires. The generator terminal voltage is shown for the first half cycle from zero to 180 degrees, when the generated voltage is positive above the aero ref erence line. This voltage is taken off the brushesand applied to the switch wtft tLTlarlii as sholn. The voltage will cause a current tc .flow trom the negative brush through the switch and load resistor and back “ Dositivcf brush. The developed voltage waveform across the load resistor is as shown. Notice that it is exactly the same as the generator terminal voltage since the resistor is connected right across the brushes. Generator terminal voltage Output voltage As the armature loop rotates through 180 degrees, the polarity of the gen erated voltage reverses. At this instant the switch is manually thrown to the other side and switches point A of the load resistor to the lower brush, which now is positive. Although the polarity of the voltage across the brushes has reversed, the polarity of the voltage across the load resistor is still the same. The action of the switch, therefore, is to reverse the polarity of the output voltage every time it changes mthe generator . In this manner the AC generated in the generator is converted to varying DC outside the generator. Generator terminal voltage Output voltage 5-14 ELEMENTARY GENERATORS The Commutator In order to convert the generated AC voltage into a varying DC voltage, the switch must be operated twice for every cycle. If the generator is putting out 60 cycles of AC each second, the switch must be operated 120 times per second to convert the AC to DC. It would be impossible to operate a switch manually at such a high speed. Designing a mechanical device to operate the switch also would be impractical. Although theoretically the switch will do the job, it must be replaced by something that will actually operate at this high speed. The slip rings of the elementary generator can be changed so they ac tuall y give the same result as the impractical mechanical switch. To do this, one slip ring is eliminated and the other is split along its axis. The ends of the coil are connected one to each of the segments of the slip ring. The segments of the split ring are insulated so that there is no electrical con- tact between segments, the shaft, or any other part of the armature. The entire split ring is known as the "commutator," and its action in converting the AC into DC is known as "commutation." You see the brushes are now positioned opposite each other, and the com- mutator segments are mounted so they are short-circuited by the brushes as the loop passes through the zero voltage points. Notice also that as the loop rotates, each conductor will be connected by means of the commutator, first to the positive brush and then to the negative brush. When the armature loop is rotated, the commutator automatically switches each end of the loop from one brush to the other each time the loop completes a half revolution. This action is exactly like that of the re- versing switch. 5-15 ELEMENTARY GENERATORS Converting AC to DC by Use of the Commutator Suppose you analyze the action of the commutator in converting the gen- erated AC into DC. In position A, the loop is perpendicular to the mag- netic field and there will be no emf generated in the conductors of the loop. As a result, there will be no current flow. Notice that the brushes are in contact with both segments of the commutator, effectively short-circuiting the loop. This short circuit does not create any problem since there is no current flow. The moment the loop moves slightly beyond position A (zero degrees), the short circuit no longer exists. The black brush is in contact with the black segment while the white brush is in contact with the white segment. As the loop rotates clockwise from position A to position B, the induced emf starts building up from zero until at position B (90 degrees) the in- duced emf is a maximum. Since the current varies with the induced emf, the current flow will also be a maximum at 90 degrees. As the loop con- tinues rotating clockwise from position B to C, the induced emf decreases until at position C (180 degrees) it is zero once again. The waveform shows how the terminal voltage of the generator varies from zero to 180 degrees. COMMUTATION — CONVERTING AC TO DC Generated Terminal Voltage 5-16 ELEMENTARY GENERATORS Converting AC to DC by Use of the Commutator (continued) Notice that in position C the black brush is slipping off the black segment and onto the white segment, while at the same time the white brush is slip- ping off the white segment and onto the black segment. In this way the black brush is always in contact with the conductor of the loop moving downward, and the white brush is always in contact with the conductor moving upward. Since the upward-moving conductor has a current flow toward the brush, the white brush is the negative terminal and the black brush is the positive terminal of the DC generator. As the loop continues rotating from position C (180 degrees) through posi- tion D (270 degrees) and back to position A (360 degrees or zero degrees) the black brush is connected to the white wire which is moving down and the white brush is connected to the black wire which is moving up. ’ As a result the same polarity voltage waveform is generated across the brushes irom 180 to 360 degrees as was generated from zero to 180 degrees. No- tice that the current flows in the same direction through the current meter even though it reverses in direction every half cycle in the loop itself. ’ The voltage output then has the same polarity at all times but varies in value, rising from zero to maximum, falling to zero, then rising to maxi- mum and falling to zero again for each complete revolution of the ar- mature loop. COMMUTATION— CONVERTING AC TO I)C 5-17 ELEMENTARY GENERATORS Improving the DC Output Before you learned about generators, the only DC voltage you were ^mil- iar with was the smooth and unvarying voltage produced, for example, by a battery. Now you find that the DC output of an elementary DC generator is very uneven— a pulsating DC voltage varying periodically from zero to a maximum. Although this pulsating voltage is DC, it is not constant enough to operate DC appliances and equipments. Therefore, the elemen- tary DC generator must be modified so that it will put out a smooth form of DC. This is accomplished by adding more coils of wire to the armature. The illustration shows a generator with a two-coil armature, with the two coils positioned at right angles to each other. Notice that the commutator is broken up into four segments, with opposite segments connected to the ends of a coil. In the position shown, the brushes connect to the white coil in which a maximum voltage is generated, since it is moving at right angles to the field. As the armature rotates clockwise, the output from the white coil starts dropping off. After an eighth of a revolution (45 degrees) the brushes slide over to the black commutator segments, whose coil is just beginning to cut into the field. The output voltage starts to pick up again, reaches a peak at 90 degrees and starts dropping off as the black coil cuts through fewer lines of force. At 135 degrees, commutation takes place once again and the brushes are again in contact with the white coil. The output voltage waveform for the entire revolution is shown super- imposed on the single coil voltage. Notice that the output never drops be low point Y. The rise and fall in voltage now is limited between Y and the maximum, rather than between zero and the maximum. This variation in the output voltage of a DC generator is known as "generator ripple." It is apparent that the output of the two- coil armature is much closer to con- stant DC than the output of the one- coil armature. 5-18 ELEMENTARY GENERATORS Improving the DC Output (continued) Even though the output of the two- coil generator is a lot closer to being constant DC than the output of the one- coil generator, there is still too much ripple in the output to make it useful for electrical equipment. To make the output really smooth, the armature is made with a large number of coils, and the commutator is similarly divided up into a large number of segments. The coils are so arranged around the armature that at every in- stant there are some turns cutting through the magnetic field at right an- gles. As a result, the generator output contains very little ripple and is for all practical purposes a constant, or "pure," DC. The voltage induced in a one-turn coil or loop is not very large. In order to generate a large voltage output, each coil on the armature of a com- mercial generator consists of many turns of wire connected in series. As a result, the output voltage is much greater than that generated in a coil having only one turn. MANY-TURN COILS INCREASE VOLTAGE OUTPUT 5-19 ELEMENTARY GENERATORS Review Now suppose you review what you have found out about the elementary gen- erator and commutation: ELEMENTARY GENERATOR — A loop of wire rotating in a magnetic field forms an elementary genera- tor and is connected to an external circuit through slip rings. ELEMENTARY GENERATOR OUT - PUT — The emf and current flow of an elementary generator reverse in polarity each time the armature loop rotates 180 degrees. The voltage output of such a generator is alter- nating current. CHANGING AC TO DC — By using a reversing switch, the AC output of an elementary generator can be changed to DC . COMMUTATOR — An automatic re- versing switch on the generator shaft which switches coil connections to the brushes each half revolution of an elementary generator. PRACTICAL GENERATOR — To smooth oiil the DC taken from a gen- erator commutator, many coils are used in the armature and more seg- ments are used to form the commu- tator. A practical generator has a voltage output which is near maxi- mum at all times and has only a slight ripple variation. 5-20 DIRECT CURRENT GENERATORS DC Generator Construction Up until now you have learned the fundamentals of generator action and the theory of operation of elementary AC and DC generators. Now you are ready to learn about actual generators and how they are constructed There are various components essential to the operation of a complete generator. Once you learn to recognize these components and become tiiei j fun 5ti° n , you will find this information useful in the troubleshooting and maintenance of generators. n!Lf" era i 0 rS r^ hether AC or DC -consist of a rotating part called a rotor and a stationary part called a "stator." In most DC generators the armature coil is mounted on the rotor and the field coils on the stator- while in most AC generators just the opposite is true— the field coils are on the rotor and the armature coil is on the stator. In either case there is relative motion between the armature and field S ,°, th f t the ar “ ature coils cut through the magnetic lines of force of the field. As a result, an emf is induced in the armature, causing a cur- rent to flow through the outside load. Since the generator supplies elec- trical power to a load, mechanical power must be put into the generator to cause the rotor to turn and generate electricity. The generator simply converts mechanical power into electrical power. Consequently, all gen- erators must have machines associated with them which will supply the mechanical power necessary to turn the rotors. These machines are called f nd ma / be ste am engines, steam turbines, electric mo- tors, gasoline engines, etc. Now suppose you find out about the construction of a typical DC generator . various com P° ne nts. Although generator construction varies widely, the basic components and their function are the same for all types. DIRECT CURRENT GENERATORS DC Generator Construction (continued) The relationship of the various components making up the generator is il- lustrated below. In assembling the generator, the fields are mounted in the stator and one end bell (not illustrated) is bolted to the stator frame. The armature is then inserted between the field poles and the end bell, with the brush assemblies mounted last. These parts will be described in de- tail on the following sheets. End Brush Screw Generator assembly and disassembly varies depending on the size, type and manufacturer; but the general method is as illustrated above. 5-22 DIRECT CURRENT GENERATORS DC Generator Construction (continued) The illustration shows a typical DC generator with the principle parts of- the stator captioned. Compare each part and its function to the corre- sponding part used in the elementary generator. Main Frame: The main frame is sometimes called the "yoke." It is the foundation of the machine and supports the other components. It also serves to complete the magnetic field between the pole pieces. Pole Pieces: The pole pieces are made of many thin layers of iron or steel called laminations, joined together and bolted to the inside of the frame. These pole pieces provide a support for the field coils and are de- signed to produce a concentrated field. By laminating the poles, eddy cur- rents, which you will learn about later, are reduced. 5-23 DIRECT CURRENT GENERATORS DC Generator Construction (continued) Field Windings: The field windings, when mounted on the pole pieces, form electromagnets which provide the magnetic field necessary for generator action. The windings and pole pieces together are often called the "field." The windings are coils of insulated wire wound to fit closely around pole pieces. The current flowing through these coils generates the magnetic field. A generator may have only two poles, or it may have a large number of even poles. Regardless of the number of poles, alternate poles will al- ways be of opposite polarity. Field windings can be connected either in series or in parallel (or "shunt" as the parallel connection is often called). Shunt field windings consist of many turns of fine wire, while series field windings are composed of fewer turns of fairly heavy wire. End Bells: These are attached to the ends of the main frame and contain the bearings for the armature. The rear bell usually supports the bearing alone while the front bell also supports the brush rigging. Brush Holder: This component is a piece of insulated material which sup- ports the brushes and their connecting wires. The brush holders are se- cured to the front end bell with clamps. On some generators, the brush holders can be rotated around the shaft for adjustment. 5-24 DIRECT CURRENT GENERATORS DC Generator Construction (continued) ^ atar e Assembly: In practically all DC generators, the armature ro- tates between the poles of the stator. The armature assembly is made up oi a shaft, armature core, armature windings and commutator. The arma- ture core is laminated and is slotted to take the armature windings. The armature windings are usually wound in forms and then placed in the slots e core. The commutator is made up of copper segments insulated L r TZ eMer and i fromthe sha ft by mica. These segments are secured rotation* Sm»n t0 preven * the™ horn slipping out under the force of rotation. Small slots are provided m the ends of the segments to which the w *? dl " g ? ar . e soldered. The shaft supports the entire armature assembly and rotates in the end bell bearings. There is a small air gap between the armature and pole pieces to prevent f, int g to ebVee “ the ari ? ature and P° le Pieces during rotation. This gap is kept to a minimum to keep the field strength at a maximum. B rushes; The brushes ride on the commutator and carry the generated voltage to the load. The brushes usually are made of a high grade of car- Sh n a nH d / re h ? ld H 1 pla c e by brush holders. The brushes Liable to slide up and down in their holders so that they can follow irregularities in the rnnntnf ° f th f! p 0 ™ 1 * 1 stator • A flexible braided conductor called a "pigtail" connects each brush to the external circuit. 5-25 DIRECT CURRENT GENERATORS DC Generator Construction (continued) You have learned that a current flow can be induced in a conductor when it cuts through a magnetic field. If a solid piece of metal cuts tlurough a mag- netic field* instead of a single wire conductor, currentalsowill be Induced inside the solid metal piece. A large, solid piece of metal large cross-section and offers little resistance to current flow. As a result, a strong current called EDDY current flows through a solid metal conductor. Since wire conductors used in motors and generators are always wound SL cores, eddy currents will be ttawmrfj! I cores just as the useful current is induced in the wires of the generator. Eddy currents flowing in the core material of rotating machinery are waste cur S slncrthe'y have no uselul purpose and only heat up the metal cores. Consequently, the machine operates at low efficiency. It is imfwrtant that eddy currents in core material be kept down to a minimum. 1S by having the cores made up of laminations, thin plates othe^ than out of one solid piece. The laminations are insulated from each other, limiting the eddy current to that which can flow in the individual iammation. The diagram illustrates the effect of laminations on limiting the magnitude of eddy currents. 5-26 DIRECT CURRENT GENERATORS Types of Armatures Armatures used in DC generators are divided into two general types. These are the "ring" type armature and the "drum" type armature. In the ring type armature, the insulated armature coils are wrapped around a hollow iron cylinder with taps taken off at regular intervals to form con- nections to the commutator segments. The ring type armature was first used in early design of rotating electrical machinery. Today the ring ar- mature is seldom used. The drum type armature is the standard armature construction today. The insulated coils are inserted into slots in the cylindrical armature core. The ends of the coils are then connected to each other at the front and back ends. As a rule, most DC armatures use form-made coils. These coils are wound by machines with the proper number of turns and to the proper shape. The entire coil is then wrapped with tape and inserted into the ar- mature slots as one unit. The coils are so inserted that the legs of the coil can only be under unlike poles at the same time. In a two-pole machine, the legs of each coil are situated on opposite sides of the core and therefore come under opposite poles. In afour-pole machine, the legs of the coils are placed in slots about one-quarter the distance around the armature, thus keeping opposite legs of the coil under unlike poles. 5-27 DIRECT CURRENT GENERATORS Types of Armature Windings Drum type armatures are wound with two types of windings, the "lap" winding and the "wave" winding. The lap winding is used for high current applications and has many paral- lel paths within the armature. As a result, there will be a large number of field poles and an equal number of brushes. The wave winding is used for high voltage applications. It has only two parallel current paths and can use only two brushes, regardless of the number of poles. The only difference between the lap and wave windings is the method used to connect the winding elements. The two drawings illustrate the essential difference between a lap and wave winding. In both windings AB connects to CD which is under the next pole. In the lap winding, CD connects back to EF which is under the same pole as AB. In the wave winding, CD is connected forward to EF which is under a pole two poles away from AB. Therefore, the essential difference is that in lap winding the connections are made lapping over each other. In wave winding the connections are made forward, so that each winding passes under every pole before it comes back to its starting pole. /4*uH4tune Position of Field Poles DIRECT CURRENT GENERATORS Types of DC Generators Most practical DC generators have electromagnetic fields. Permanent- magnet fields are used only in very small generators called "magnetos." To produce a constant field for use in a generator, the field coils must be connected across a DC voltage source. (AC current flow in a field coil does not produce a constant field and, therefore, an AC voltage source can- not be used.) The DC current in the field coils is called the "excitation current" and may be supplied from a separate DC voltage source, or bv utilizing the DC output of the generator itself. DC generators are classified according to the manner in which the field is supplied with excitation current. If the field is supplied with current from an external source, the generator is said to be "separately-excited." However, if some of the generator output is used to supply the field cur- rent, it is said to be "self-excited". The circuit of the generator armature and field coils determines its type and affects its performance. Various generators utilize the three basic DC circuits — series, parallel and series- parallel. Symbols, as illustrated below, are used to represent the arma- ture and field coils in the various generator circuits. Separately-excited DC generators have two circuits, each entirely inde- pendent of the other: the field circuit consisting of the field coils connected across a separate DC source, and the armature circuit consisting of the armature coil and the load resistance. (When two or more field coils are connected in series with one another, they are represented by a single symbol.) The two circuits of a separately-excited generator are illustrated below, showing the current flow through the various parts of the circuit. SEPARATELY- EXCITED DC GENERATORS 5-29 DIRECT CURRENT GENERATORS Separately-Excited DC Generators In a seDaratelv-excited DC generator, the field is independent of the arma- ture, since it is supplied with current from either another g en erator (ex- citeri an amplifier or a storage battery. The separately-excited fie provides a very sensitive control of the power output of / h e generator since the field current is independent of the load current. With a slight change in the field current, a large change in the load current will result. The separately-excited generator is used mostly in automatic motor control systems. In these systems the field power is controlled by an am- plifier and the output of the generator supplies the armature current which drives the motor. The motor is used to position a gun turret, a search light or any other heavy mechanism. Separately-Excited DC Generators Separately-Excited DC Generator Radar Antenna Field ► current supply. Variable current and polarity » Field Coil Generator DC \ Output f DC {Gener-J Motor] DC Supply from amplifier s\ cr* 5-3 DIRECT CURRENT GENERATORS Self -Excited DC Generators Self-excited generators use part of the generator's output to supply ex- citation current to the field. These generators are classified according to the type of field connection used. In a "series" generator, the field coils are connected in series with the armature, so that the whole armature current flows through both the field and the load. If the generator is not connected across a load, the circuit is incomplete and no current will flow to excite the field. The series field contains relatively few turns of wire. "Shunt" generator field coils are connected across the armature circuit, forming a parallel or "shunt" circuit. Only a small part of the armature current flows through the field coils, the rest flowing through the load. Since the shunt field and the armature form a closed circuit independent of the load, the generator is excited even under "no load" conditions — with no load connected across the armature. The shunt field contains many turns of fine wire. A "compound" generator has both a series and a shunt field, forming a series-parallel circuit. Two coils are mounted on each pole piece, one coil series-connected and the other shunt-connected. The shunt field coils are excited by only a part of the armature current, while the entire load current flows through the series field. Therefore, as the load current in- creases, the strength of the series field is increased. Self-excited DC Generators. , \ Series ■jv V“ Field *r-M A rmaturel'tl fly— e S. „ t 1 Load Shunt Field Q SERIES GENERATOR Connection Symbols: Armature - A-l, A- 2 Shunt Field Series Field F-l, F-2 S-l, S-2 I Load 4/WVH SHUNT GENERATOR Load M/WVH 0 COMPOUND GENERATOR 5-31 DIRECT CURRENT GENERATORS Self -Excited DC Generators (continued) Shunt field coils, which connect directly across the generator output volt- age, are constructed of many turns of small wire so that the coil resistance will be great enough to limit the current flow to a low value. Since the shunt field current is not used to supply the load, it is necessary to keep it to as low a value as possible. If the shunt field of a compound generator is connected across both the se- ries field and the armature, the field is called a "long shunt" field. If the shunt field is connected just across the armature, the field is called a "short shunt" field. The characteristics of both bypes of shunt connections are practically the same. Series field coils are constructed of fewer turns of heavier wire and de- pend on the large current flow to the load resistance for their magnetic field strength. They must have a low resistance since they are in series with the load and act as a resistor to drop the voltage output of the gen- erator. A comparison of the connections used for the various generator circuits is outlined below: 5-32 DIRECT CURRENT GENERATORS Seif -Excited DC Generators (continued) Almost all of the DC generators used for lighting and power are the self- excited type, in which armature current is used to excite the field. How- ever, if the original field excitation depends upon this armature current, and no current is induced in the armature coil unless it moves through a magnetic field, you may wonder how the generator output can build up. In other words, if there is no field to start with (since no current is flowing through the field), how can the generator produce an emf? Actually the field poles retain a certain amount of magnetism called the "residual magnetism," from a previous generator run, due to the magne- tism characteristics of their steel structure. When the generator starts turning, an original field does exist which, although very weak, will still induce an emf in the armature. This induced emf forces current through the field coils, reinforcing the original magnetic field and strengthening the total magnetism. This increased flux in turn generates a greater emf which again increases the current through the field coils. This action in- creases until the machine attains its normal field strength. All self- excited generators build up in this manner. The build-up time normally is 20 to 30 seconds. The graph shows how generator voltage and field cur- rent build up in a shunt generator. Remember, the output of a generator is electrical power. A generator al- ways has to be turned by some mechanical means — the prime mover. "Build up" in a generator does not refer to its mechanical rotation, it re- fers to its electrical output. Generator Voltage “SuiCd-ctfe . . . Shunt Generator 3 90 % 80 & 70 3 60 o > 50 2 40 S 30 § 20 O 10 5-33 Field current (amperes) DIRECT CURRENT GENERATORS Sell -Excited DC Generators (continued) Sometimes generators willnot build up. When this happens, one of several things may be wrong. There may be too little or no residual magnetism. To provide the initial field necessary, the generator must be excited by an external DC source. This is called "flashing the field." When flashing the field, it is important to have the externally produced field of the same polarity as the residual magnetism. If these polarities are opposed, the initial field will be further weakened and the generator will still not build up. The generator will not build up if the shunt field connections have been re- versed. By reversing them again, the generator will build up properly. Often a rheostat is connected in series with the shunt field, to control the field current. If this rheostat adds too much resistance to the circuit at first, the field current will be too small for a proper build-up. F inall y, if the field coil circuit has become "open," so the circuit is not complete, the generator will not build up. The break or open must be found and repaired. ;jHjijj:j;jjj{;;;j{H^ OPEN FIELD CIRCUIT mam REVERSED FIELD CONNECTION 5-34 DIRECT CURRENT GENERATORS The Series Generator In the series generator, the armature, the field coils and the external cir- cuit are all in series. This means that the same current which flows through the armature and external circuit also flows through the field coils. Since the field current, which is also the load current, is large, the re- quired strength of magnetic flux is obtained with a relatively small number of turns in the field windings. The illustration shows the schematic of a typical DC series generator. With no load, no current can flow and therefore very little emf will be in- duced in the armature — the amount depending upon the strength of the resi- dual magnetism. If a load is connected, current will flow, the field strength will build up and, consequently, the terminal voltage will increase. As the load draws more current from the generator, this additional current in- creases the field strength, generating more voltage in the armature wind- ing. A point is soon reached (A) where further increase in load current does not result in greater voltage, because the magnetic field has reached the saturation point. Beyond point A, increasing the load current decreases voltage output due to the increasing voltage drop across the resistance of the field and armature. The series generator always is operated beyond this point of rapidly drop- ping terminal voltage (between A and B), so that the load current will re- main nearly constant with changes in load resistance. This is illustrated by the voltage graph. For this reason, series generators are called "con- stant current generators." Series generators formerly were used as constant current generators to operate arc lamps. At the present time, they are not used aboard ships in the Navy. iHfcB Series Generator Series Generator Operating portion of curve Load current CHARACTERISTIC CURVE 5-35 DIRECT CURRENT GENERATORS The Shunt Generator The shunt generator has its field winding connected in shunt (or parallel) with the armature. Therefore the current through the field coils is deter- mined by the terminal voltage and the resistance of the field. The shunt field windings have a large number of turns, and therefore require a rela- tively small current to produce the necessary field flux. When a shunt generator is started, the buildup time for rated terminal volt- age at the brushes is very rapid since field current flows even though the external circuit is open. As the load draws more current from the arma- ture, the terminal voltage decreases because the increased armature drop subtracts from the generated voltage. The illustration shows the sche- matic diagram and characteristic curve for the shunt generator. Observe that over the normal operating region of no load to full load (A-B), the drop in terminal voltage, as the load current increases, is relatively small. As a result, the shunt generator is used where a practically constant voltage is desired, regardless of load changes. If the load current drawn from the generator increases beyond point B, the terminal voltage starts dropping off sharply. The generator is never run beyond point B. The terminal volt- age of a shunt generator can be controlled by varying the resistance of a rheostat in series with the field coils. The Shunt Generator DIRECT CURRENT GENERATORS The Compound Generator A compound generator is a combined series and shunt generator. There are two sets of field coils— one in series with the armature and one in parallel with the armature. One shunt coil and one series coil are always mounted on a common pole piece, and sometimes enclosed in a common covering. H the series field is connected so that its field aids the shunt field, the gen- erator is called "cumulatively" compound. If the series field opposes the shunt field, the generator is called "differentially" compound. Also, as ex- plained before, the fields may be connected "short shunt" or "long shunt, " depending on whether the shunt field is in parallel with both the series field and the armature, or just the armature. The operating characteristics for both types of shunt connections are practically the same. Compound Generators Compound generators were designed to overcome the drop in terminal voltage which occurs in a shunt generator when the load is increased This voltage drop is undesirable where constant voltage loads, such as lighting systems, are used. By adding the series field, which increases the strength of the total magnetic field when the load current is increased, the voltage drop due to the added current flowing through the armature re- sistance is overcome, and constant voltage output is practically attained. 5-37 DIRECT CURRENT GENERATORS The Compound Generator (continued) The voltage characteristics of the cumulative compound generator depend on the ratio of the turns in the shunt and series field windings. If the series windings are so proportioned that the terminal voltage is practically con- stant at all loads within its range, it is "flat-compounded." Usually in these machines the full-load voltage is the same as the no-load voltage, and the voltage at intermediate points is somewhat higher. Flat-compounded gen- erators are used to provide a constant voltage to loads a short distance away from the generator. An "overcompounded" generator has its series turns so selected that the full -load voltage is greater than the no-load volt- age. These generators are used where the load is some distance away. The increase in terminal voltage compensates for the drop in the long feeder lines, thus maintaining a constant voltage at the load. When the rated volt- age is less than the no-load voltage, the machine is said to be "undercom- pounded." These generators are seldom used. Most cumulative compound generators are overcompounded. The degree of compounding is regulated by placing a low resistance shunt called a "diverter" across the series field terminal as shown. The terminal voltage can be controlled by varying the field rheostat in series with the shunt field. In a differentially compounded generator the shunt and series fields are in opposition. Therefore the dif- ference, or resultant field, becomes weaker and the terminal voltage drops very rapidly with increase in load current. The characteristic curves for the four types of compound generators are illustrated. The Compound Generator) 5-38 DIRECT CURRENT GENERATORS Commutation When you studied the elementary DC generator, you learned that the brushes are positioned so that they short-circuit the armature coil when it is not cutting through the magnetic field. At this instant no current flows and there is no sparking at the brushes (which are in the act of slip- ping from one segment of the commutator to the next.) Piopei Commutation If the brushes are moved a few degrees, they short-circuit the coil when it is cutting through the field. As a result, a voltage will be induced in the short-circuited coil, and a short-circuit current will flow to cause sparking at the brushes. This condition is undesirable since the short circuit cur- rent may seriously damage the coils and burn the commutator. This situ- ation can be remedied by rotating both brushes so that commutation takes place when the coil is moving at right angles to the field. DC generators operate efficiently when the plane of the coil is at right angles to the field at the instant the brushes short the coil. This plane which is at right angles to the field is known as the "plane of commuta- tion" or "neutral plane." The brushes will short-circuit the coil when no current is flowing through it. 5-39 DIRECT CURRENT GENERATORS Armature Reaction You know that for proper commutation, the coil short-circuited by the brushes should be in the neutral plane. Suppose you consider the operation of a simple two-pole DC generator. The armature is shown in a simplified view with the cross section of its coil represented as little circles. When the armature rotates clockwise, the sides of the coil to the left will have current flowing out of the paper and the sides of the coil to the right will have current flowing into the paper. The field generated around each side of the coil is also shown. Now you have two fields in existence — the main field and the field around each coil side. The diagram shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation. If the brushes remain in the old neutral plane, they will be short-circuiting coils which have voltage induced in them. Consequently, there will be arc- ing between the brushes and commutator. To prevent this, the brushes must be shifted to the new neutral plane. The reaction of the armature in displacing the neutral plane is known as "ar- mature reaction.” Aimatuie Reaction 5-40 DIRECT CURRENT GENERATORS Compensating Windings and Interpoles Shifting the brushes to the advanced position of the neutral plane does not completely solve the problems of armature reaction. The effect of arma- ture reaction varies with the load current. Therefore, every time the load current varies, the neutral plane shifts, meaning the brush position will have to be changed. In small machines the effects of armature reaction are minimized by me- chanically shifting the position of the brushes. In larger machines more elaborate means are taken to eliminate armature reaction, such as using compensating windings or interpoles. The compensating windings consist of a series of coils embedded in slots in the pole faces. The coils are con- nected in series with the armature so that the field they generate will just cancel the effects of armature reaction, for all values of armature current. As a result the neutral plane remains stationary and, once the brushes have been set correctly, they do not have to be moved again. Another way to minimize the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The inter- poles have a few turns of large wire connected in series with the armature. The field generated by the interpoles just cancels the armature reaction for all values of load current and improves commutation. Connecting Armature Reaction COMPENSATING WINDINGS Shunt Interpoles or Compensating j Windings To Load 5-41 DIRECT CURRENT GENERATORS Review of DC Generators GENERATOR CLASSIFICATION - DC generators are classified ac- cording to the method of field ex- citation used. Separately-excited generators use an outside source of DC current to magnetize the fields. Self-excited generators use the output of the generator it- self to excite the field. Self-excited generators are fur- ther divided into classifications, depending on the field winding connections. SERIES GENERATOR — The field has few turns of heavy wire and connects in series with the arma- ture. It is operated on the con- stant current part of its voltage output curve to provide a constant current output. SHUNT GENERATOR — The field has many turns of small wire and connects directly across the ar- mature. The output voltage drops as the load current increases. COMPOUND GENERATOR — The field has two sets of windings— a shunt field and a series field. The combined effect of the two fields makes the output voltage nearly constant regardless of the load current. 5-42 DIRECT CURRENT GENERATORS Review of DC Generators (continued) PROPER COMMUTATION — The brushes of a DC generator should short out the commutator segments of the armature loop in which no emf is being generated at the moment of commutation. At this moment, the generating conductors of the loop are moving parallel to the lines of force in the field. COMMUTATOR SPARKING — If the brushes short out the commutator segments whose armature conduc- tors are not moving parallel to the lines of force in the field, the gener- ated emf is short-circuited, causing arcing at the brushes. Shifting the brushes reduces this arcing. ARMATURE REACTION — Current flow in the armature coil generates a magnetic field at right angles to that of the generator field poles. The resultant total field shifts the neutral plane. COMPENSATING WINDINGS — Windings placed in the field pole faces, carrying the same current as the armature coil but in the opposite directions, counteract the armature field. INTERPOLES — Small poles mounted between the main field windings, to generate a field exactly opposite to that of the armature coil. 5-43 DIRECT CURRENT MOTORS Converting Electrical Power to Mechanical Power DC motors and DC generators have essentially the same components and are very similar in outward appearance. They differ only in the way they are used. In a generator, mechanical power turns the armature and the moving armature generates electrical power. In a motor, electrical power forces the armature to turn and the moving armature, through a mechani- cal system of belts or gears, turns a mechanical load. A DC generator converts mechanical energy to electrical energy. A DC motor converts electrical energy into mechanical energy. THEIR DIFFERENCE LIES IN THE WAY THEY ARE USED DIRECT CURRENT MOTORS Converting Electrical Power to Mechanical Energy (continued) How a DC motor works is not completely new to you. In studying meters you learned that a galvanometer has a coil suspended between the poles of a horseshoe magnet. When a current flows through the coil, the coil itself acts as a magnet, and the coil is moved by the force between the two mag- netic fields. This is the principle of operation for all DC motors, from the smallest to the largest. Therefore, to understand practical motors, you can start with the most elementary — a single turn coil suspended be- tween the poles of a magnet. 5-45 DIRECT CURRENT MOTORS Fleming and Lenz Fleming discovered the method for determining the direction of rotation of a motor if the direction of the current is known. The importance of this information cannot be overestimated, as you will see when you learn more about the principles which govern the operation of the numerous motors and generators in use today. Fleming found that there is a definite relation between the direction of the magnetic field, the direction of current in the conductor, and the direction in which the conductor tends to move. This relationship is called Fleming Right Hand Rule for Motors. If the thumb, index finger, and third finger of the right hand are extended at right angles to each other, and if the hand is so placed that the index finger points in the direction taken by the flux lines of the magnetic field, then the thumb will point out the direction of motion of the conductor and the third finger will point in the direction taken by the current through the conductor. Obviously, if the direction of the magnetic field is not known but the motion of the' conductor and the direction of the current through the conductor are known, the index finger must point in the direction of the magnetic field, provided the right hand is placed in the proper position. The diagram below illustrates Fleming's Right Hand Rule for Motors. If you use this rule, you can always determine the direction of rotation of motors, provided you know the direction of the current. Motion of conductor RIGHT HAND RULE FOR MOTORS 5-46 DIRECT CURRENT MOTORS Fleming and Lenz (continued) You have learned about the laws which were discovered by Fleming. Lenz's Law is the next basic law with which you will come in contact. An under- s anding of this law will be a tremendous help in your understanding of the whole field of motors and generators. A conductor which carries a current is surrounded by a magnetic field. This is true even if the current is the result of an induced emf. Figure 1 below shows a conductor at rest in a magnetic field. No emf is induced and no current flows because the conductor is stationary. In Figure 2 the conductor is pushed downward. The result is an induced emf which produces a current flow in the conductor. Since a magnetic field surrounds every conductor which carries a current, the conductor will have a mag- ueiii; field of its own because of the induced emf and resulting current flow. This magnetic field will be set up in the direction indicated in Figure 3. Two magnetic fields now exist; one from the current through the conductor and the other from the magnet. Since magnetic fields never cross, the lines of the two fields either crowd together or cancel each other out, producing either strong or weak re- sultant fields, respectively. In Figure 4 the two magnetic fields are op- posing, and therefore cancel each other out. The result is a weak mag- netic field above the conductor. Figure 5 shows that the magnetic fields below the wire are in the same direction and therefore additive. Downward push N LENZ'S LAW Action between conductor and magnetic fields Reenforcement 5-47 DIRECT CURRENT MOTORS Fleming and Lenz (continued) The field of the magnet is, then, distorted by the field which surrounds the current-carrying wire. A weak resultant field exists above the wire, and a strong resultant field exists below the wire. Remember that flux lines tend to push each other apart. The diagram below shows that the flux lines under the conductor, in pushing each other apart, tend to push the conduc- tor up, whereas those above the conductor tend to push each other down. Since, however, there are more flux lines below the conductor than there are above, the upward push is greater, and the conductor tends to move in the upward direction. DOWNWARD PUSH UPWARD REACTION OF WIRE Before going on, it is well to summarize the above information, as follows: 1. The "straight" magnetic field which exists between the poles of the mag- net is distorted by the circular magnetic field which surrounds the current-carrying conductor. 2. A downward force is applied by a push on the conductor. 3. An upward force results from the distorted field. These facts tell you that if you push a conductor, moving it across a mag- netic field, an emf is induced in the conductor. This emf causes current to flow through the conductor, setting up a new magnetic field which tries to move the conductor back against the push. This, in effect, is a genera statement of Lenz's Law. Lenz found that in all cases of electromagnetic induction, the direction of the induced emf is such that the magnetic field set up by the resulting current tends to stop the motion which is producing the emf. The induced emf just described actually opposes the applied line voltage. The induced emf which develops in the rotating armature of a motor is called a counter emf. This counter emf is of tremendous importance in motor operation. Motor armature resistances are usually extremely low; frequently less than one ohm. If the usual 110- or 220-volt line source is applied to an armature, huge currents flow and burn-out occurs almost im- mediately. However, since the counter emf (cemf) always opposes the line voltage source, an automatic current-limiter is always present to cut armature current to safe limits. 5-48 DIRECT CURRENT MOTORS DC Motor Principles The elementary DC motor is constructed similarly to the elementary DC generator. It consists of a loop of wire that turns between the poles of a magnet. The ends of the loop connect to commutator segments which in turn make contact with the brushes. The brushes have connecting wires going to a source of DC voltage. Keep in mind the action of the meter movement, and compare it to that of the elementary DC motor. With the loop in position 1, the current flowing through the loop makes the top of the loop a north pole and the underside a south pole, according to the left-hand rule. The magnetic poles of the loop will be attracted by the corresponding opposite poles of the field. As a result, the loop will rotate clockwise, bringing the unlike poles together. When the loop has rotated through 90 degrees to position 2, commutation takes place, and the current through the loop reverses in direction. As a result, the magnetic field generated by the loop reverses. Now like poles face each other, which means they will repel each other, and the loop con- tinues rotating in an attempt to bring unlike poles together. Rotating 180 degrees past position 2, the loop finds itself in position 3. Now the situa- tion is the same as when the loop was back in position 2. Commutation takes place once again and the loop continues rotating. This is the funda- mental action of the DC motor. DIRECT CURRENT MOTORS Commutator Action in a DC Motor It is obvious that the commutator plays a very important part in the oper- ation of the DC motor. The commutator causes the current through the loop to reverse at the instant unlike poles are facing each other. This causes a reversal in the polarity of the field; repulsion exists instead of attraction, and the loop continues rotating. In a multi- coil armature, the armature winding acts like a coil whose axis is perpendicular to the main magnetic field and has the polarity shown be- low. The north pole of the armature field is attracted to the south pole of the main field. This attraction exerts a turning force on the armature, which moves in a clockwise direction. Thus a smooth and continuous torque or turning force is maintained on the armature due to the large number of coils. Since there are so many coils close to one another, a re- sultant armature field is produced that appears to be stationary. 5-50 DIRECT CURRENT MOTORS Armature Reaction Since the motor armature has current flowing through it, a magnetic field will be generated around the armature coils as a result of this current, This armature field will distort the main magnetic field — the motor has "armature reaction" just as the generator. However, the direction of dis- tortion due to armature reaction in a motor is just the opposite of that in a generator. In a motor, armature reaction shifts the neutral commutating plane against the direction of rotation. To compensate for armature reaction in a motor, the brushes can be shifted backwards until sparking is at a minimum. At this point, the coil being short-circuited by the brushes is in the neutral plane and no emf is induced in it. Also, armature reaction can be corrected by means of com- pensating windings and inter poles, just as in a generator, so that the neu- tral plane is always exactly between the main poles and the brushes do not have to be moved once they are properly adjusted. 5-51 DIRECT CURRENT MOTORS Reversing the Direction of Motor Rotation The direction of rotation of a motor depends upon the direction of the field and the direction of current flow in the armature. Current flowing through a conductor will set up a magnetic field about this conductor. The direc- tion of this magnetic field is determined by the direction of current flow. If the conductor is placed in a magnetic field, force will be exerted on the conductor due to the interaction of its magnetic field with the main mag- netic field. This force causes the armature to rotate in a certain direction between the field poles. In a motor, the relation between the direction of the magnetic field, the direction of current in the conductor, and the direc- tion in which the conductor tends to move is expressed in the right-hand rule for motor action, which states: Place your right hand in such a posi- tion that the lines of force from the north pole enter the palm of the hand. Let the extended fingers point in the direction of current flow in the con- ductor; then the thumb, placed at right angles to the fingers, points in the direction of motion of the conductor. If either the direction of the field or the direction of current flow through the armature is reversed, the rotation of the motor will reverse. However, if both of the above two factors are reversed at the same time, the motor will continue rotating in the same direction. Ordinarily a motor is set up to do a particular job which requires a fixed direction of rotation. However, there are times when you may find it nec- essary to change the direction of rotation. Remember that you must re- verse the connections of either the armature or the field, but not both. 5-52 DIRECT CURRENT MOTORS Counter Electromotive Force In a DC motor, as the armature rotates the armature coils cut the mag- netic field, inducing a voltage or electromotive force in these coils. Since this induced voltage opposes the applied terminal voltage, it is called the "counter electromotive force," or "counter-emf." This counter-emf de- pends on the same factors as the generated emf in the generator— the speed and direction of rotation, and the field strength. The stronger the field and the faster the rotating speed, the larger will be the counter-emf. However, the counter-emf will always be less than the applied voltage be- cause of the internal voltage drop due to the resistance of the armature coils. The illustration represents the counter-emf as a battery opposing the applied voltage, with the total armature resistance shown symbolically as a single resistor. VOLTAGE SOURCE ARMATURE DROP - COUNTER-EMF ' k<- - Ui V' ' 1 What actually moves the armature current through the armature coils is the difference between the voltage applied to the motor (E a ) minus the counter-emf (Eg). Thus E a - E c is the actual voltage effective in the ar- mature and it is this effective voltage which determines the value of the armature current. Since ^generally I = ^-from Ohm’s law, in the case of the DC motor, ^ — . Also, since according to Kirchhoff's Second Law, the sum of the voltage drops around any closed circuit must equal the sum of the applied voltages, then E a = E c + I a R a . 5-53 DIRECT CURRENT MOTORS Counter Electromotive Force (continued) The internal resistance of the armature of a DC motor is very low, usually less than one ohm. If this resistance were all that limited the ar- mature current, this current would be very high. For example, if the armature resistance is 1.0 ohm and the applied line voltage is 230 volts, the resulting armature current, according to Ohm's law, would be: t = -|M = 230 amps. This excessive current would completely burn out the armature. However, the counter-emf is in opposition to the applied voltage and lirnits the value of armature current that can flow. If the counter-emf is 220 volts, then the effective voltage acting on the armature is the difference between the terminal voltage and the counter-emf: 230 - 220 - 10 volts. 10 in The armature current is then only 10 amps: ^ = R = ~ = 10 am P s - v d When the motor is just starting and the counter-emf is too small to limit the current effectively, a temporary resistance called the "starting re- sistance" must be putin series with the f mature, to keep the current flow within safe limits. As the motor speeds up, the counter-emf increases and the resistance can be gradually reduced, allowing a further increase in speed and counter-emf. At normal speed, the starting resistance is com- pletely shorted out of the circuit. 5-54 DIRECT CURRENT MOTORS Speed Depends On Load The torque a motor develops, to turn a certain load, depends on the amount of armature current drawn from the line. The heavier the load, the more torque required, and the greater the armature current must be. The lighter the load, the less torque required, and the smaller the armature current must be. The armature voltage drop (I a R a ) and the counter-emf (E c ) must always add to equal the applied terminal voltage (Et)— Et = I a R a + ®c* ^hice the terminal voltage (Et) is constant, the sum of the voltage drop and the counter-emf (I a R a + E c ) must be constant too. If a heavier load is put on the motor, it slows down. This reduces the counter-emf, which is de- pendent on the speed. Since E c + I a R a is constant, and E c is less, then IaR a must be more. The armature resistance is not changed, so the cur- rent must have increased. This means the torque developed is greater, and the motor is able to turn the heavier load at a slower speed. There? fore, you see the speed of a DC motor depends upon the load it is driving. U»«E TW* HOW SPEED VARIES WITH TORQUE REQUIREMENTS SMALL TORQUE (LIGHT LOAD) \ SMALL CURRENT (Et = I a R a + E c ) I SPEED / 5-55 DIRECT CURRENT MOTORS Changing Motor Speed The speed of a DC motor depends on the strength of the magnetic field and the value of the applied voltage, as well as the load. If the strength of the field is decreased, the motor must speed up to maintain the proper amount of counter-emf. If the field circuit should become open, only the residual magnetism is left and the motor speed increases dangerously, trying to maintain the counter-emf necessary to oppose the applied voltage. With a light load or no load, an open field circuit can cause the motor to turn so fast it will tear itself apart— the commutator segments and other parts will fly out and may cause serious injury to personnel. Always be sure the field circuit is closed before running a DC motor, and always be sure the starting resistance is set to maximum before terminal voltage is applied. The motor speed may be controlled by controlling the field strength with a field rheostat, or by controlling the voltage applied to the armature with an armature rheostat. Increasing the resistance in the armature circuit has the same effect as decreasing the voltage supply to the motor, which is to decrease the speed. This method is seldom used because a very large rheostat is necessary and also because the starting torque is lowered. In- creasing the resistance in the field circuit decreases the field current and therefore the field strength. A decreased field strength means the motor must turn faster to maintain the same counter-emf. To summarize, the speed of rotation of a DC motor depends on the field strength and the armature voltage. METHODS OF SPEED CONTROL 5-56 DIRECT CURRENT MOTORS Shunt Motors In a shunt connected motor, the field is connected directly across the line and therefore is independent of variations in load and armature current. The developed torque varies with the armature current. If the load on the motor increases, the motor slows down, reducing the counter- emf which depends on the speed as well as the constant field strength. The reduced counter-emf allows the armature current to increase, thereby furnishing the heavier torque needed to drive the increased load. If the load is de- creased, the motor speeds up, increasing the counter-emf and thereby de- creasing the armature current and the developed torque. Whenever the load changes, the speed changes until the motor is again in electrical bal- ance that is, until E c + ^Ra = Ej again. In a shunt motor, the variation of speed from no-load to normal or "full" load is only about 10 percent of the no-load speed. For this reason, shunt motors are considered constant speed motors. When a shunt motor is started, a starting resistance must be connected in series with the armature to limit the armature current until the speed builds up the necessary counter-emf. Since the starting current is small, due to this added resistance, the starting torque will be small. Shunt motors are usually used where constant speed under varying load is de- sired, and where it is possible for the motor to start under light or no load conditions. TO LINE TT TO LINE THE DC SHUNT MOTOR 5-57 DIRECT CURRENT MOTORS Series Motors Since DC motors are electrically the same as DC generators, they are both classified according to their field connections. The series motor has its field connected in series with the armature and the load, as shown. The field coil consists of a few turns of heavy wire, since the entire armature current flows through it. If the load increases, the motor slows down and the counter-emf decreases, which allows the current to increase and supply the heavier torque needed. The series motor runs very slowly with heavy loads and very rapidly with light loads. If the load is completely removed, the motor will speed dangerously and fly apart, since the current required is very small and the field very weak, so that the motor cannot turn fast enough to generate the amount of counter- emf needed to restore the balance. Series motors must never be run under no-load conditions, and they are seldom used with belt drives where the load can be removed. Also, you can see that series motors are variable speed motors— that is, their speed changes a great deal when the load is changed. For this reason series motors are seldom used where a constant operating speed is needed, and are never used where the load is intermittent — where the load changes frequently or is put on and taken off while the motor is running. 5-58 DIRECT CURRENT MOTORS Series Motors (continued) The torque— the turning force— developed by any DC motor depends on the armature current and the field strength. In the series motor, the field strength itself depends on the armature current, so that the amount of torque developed depends doubly on the amount of armature current flow- ing. When the motor speed is low, the counter-emf is, of course low and the armature current is high. This means the torque will be very high when the motor speed is low or zero, such as when the motor is starting. The series motor is said to have a high starting torque. There are special jobs which require a heavy starting torque and the high rate of acceleration this heavy torque allows. Such applications are cranes, electric hoists and electrically powered trains and trolleys. The motors used in these machines are always series motors, because the loads here are very heavy at start and then become lighter once the machine is in motion. THE DC SERIES MOTOR TREMENDOUS STARTING TORQUE RAPID ACCELERATION HEAVY LOAD - High Torque Low Speed LIGHT LOAD - Low Torque High Speed 5-59 DIRECT CURRENT MOTORS Compound Motors A compound motor is a combination series and shunt motor. The field consists of two separate sets of coils. One set, whose coils are wound with many turns of fine wire, is connected across the armature as a shunt field. The other set, whose coils are wound with few turns of heavy wire, is con- nected in series with the armature as a series field. The characteristics of the compound motor combine the features of the series and shunt motors. Cumulatively compound motors, whose series and shunt fields are connected to aid each other, are the most common. In a cumulatively compound motor , an increase in load decreases the speed and greatly increases the developed torque. The starting torque is also large. The cumulative compound motor is a fairly constant speed motor with excellent pulling power on heavy loads and good starting torque. In a differentially compound motor, the series field opposes the shunt field and the total field is weakened when the load increases. This allows the speed to increase with increased load, up to a safe operating point. The starting torque is very low, and the differentially compound motor is rarely used. Shunt Field 5-60 DIRECT CURRENT MOTORS Comparison of DC Motor Characteristics Cl ? rac 1 teristics <* the different types of DC motors can be simunarized by drawing a graph which shows how the speed varies with that^he^JL 1 ^^ tt l° t0r : Th * graph contains four curves. Notice ^ i ^ the Shunt motor varies the least as the torque require- ments of the load increase. On the other hand, the series motor speed greatly drops as the torque requirements increase. The cumulatively . co ™P° und *?° tor *>“ speed characteristics between the series and > shunt machines. Observe that the more heavily compounded (the greater compared 10 sh “* *■"■*» — i «-S£ The second graph shows how the developed torque varies with armature current for the different motors of the same horsepower rating. The torque curve for the shunt motor is a straight line because the field re- ThP^nrv!^ 114 ’ ?K d the 1 tor< * ue varies directly with the armature current. The curves for the series and compound motors show that above the full load or normal operating current, the developed torque is much greater thp n ipr l he S j Unt motor - Below the ful1 load current, the field strength of the series and compound machines have not reached their full valueand therefore, the developed torque is less than in the shunt machine. 5-61 DIRECT CURRENT MOTORS Review of DC Motors DC MOTOR PRINCIPLE — Current flow through the armature coil causes the armature to act as a magnet. The armature poles are attracted to field poles of opposite polarity, causing the armature to rotate. nr MOTOR COMMUTATION — The commutator reverses the armature current at the moment when unlike poles of the armature and field are facing each other, reversing the polar- ity of the armature field. Like poles of the armature and field then repel each other causing continuous armature rotation. DC MOTOR COUNTER ELECTROMO- TIVE FORCE — The rotating armature coil of a DC motor generates an elec- tromotive force which opposes the ap- plied voltage. This generated counter- emf limits the flow of armature current DC MOTOR SPEED CONTROLS — The speed of a DC motor can be varied with a variable resistance connected either in series with the field coil or in series with the armature coil. Increasing shunt field circuit resistance increases motor speed, while increasing the ar- mature circuit resistance decreases motor speed. ARMATURE REACTION — The arma- ture field causes distortion of the main field in a motor, causing the neutral plane to be shifted in the direction op- posite to that of armature rotation. Interpoles, compensating windings, and slotted pole pieces are used to mini- mize the effect of armature reaction on motor operation. MOTOR 5-62 DIRECT CURRENT MOTORS Review of DC Motors (continued) SERIK»S MOTORS — TKe field windings are connected in series with the arma- ture coil and the field strength varies with changes in armature current. When its speed is reduced by a load, the series motor develops greater torque, apd its starting torque is greater than that of other types of DC motors. SHUNT MOTORS — The field windings are connected in parallel across the armature coll and the field strength is independent of the armature current. Shunt motor speed varies only slightly with changes in load and the starting torque is less than that of other types of DC motors. COMPOUND MOTORS — One set of field windings is connected in series with the armature, and one set is parallel- connected. The speed and load characteristics can be changed by con- necting the two sets of fields to either aid or oppose each other. MOTOR REV ERSAL — The direction of rotation of a DC motor can be reversed by reversing the field connections or by reversing the armature connections. 5-63 DC MOTOR STARTERS DC Starters and Controllers ctnrivincr DC motors you learned that the armature resistance is very i es ? t£n oSTohm. If this resistance were the only opposi- tion to current flow, the armature current would be When the motor is running the covmter-end generated ^ h « r ^u^ C ur- S r HS S K ctnmter-emf is zero , or very low and the starting current would be very high if it was not limited 7 in some way. To prevent this high starting current, which would Saee the armature windings and commutator, a resistance called Oms " starting resistor" is put in series with the armature at starting. As th Se cinttr-emf Increases, the starting resistor is gradnaU, shorted out of the circuit. The complete starting resistor assembly is called a ^ ^^rter uS^Uy deludes 0 in^sTi the fie Id c^cuite tecome sistLice £ automaticaUy stops. When the DC starter is constructed so it can also control the op erating speed of the motor, it is called a controller. There are various types of starters, some manually control^ and some automatic. Usually the starting current is Umitedto ab°utl50perce f.iii-injirf current. There are some small DC motors wno&e tures contain many turns of fine wire, offering enough resistance to toe currenMlow so that a starter is not required. However, all large DC motors require some type of starter or controller. is essentially this. one to armature andfthe other to ,,,. e l! 2f* The . holding coil is m series with the field and armature so that from the HnT * V" th ® 0ff P° sition > th e armature and field are disconnected abl^to biiw'un ar f 1 from ° ne P° int to the next, the motor is ture ^rrent wfif Itf? count e r -emf in step with the increase in arma- ture current. When the arm is in the running position, the resistance is S removed from the armature and field circuit. The arm is ^th the Smarnip ?° Sition ** the holding coil, which is in series -a ^ J ? d therefore energized by the armature current. When tteSfrt nf f^°i, Ve il fr0m u® “ oto . r >the armature current drops, weakening Stinn t? the holding coil. The arm is released and moves to the off po- the bnp iho Pmg th f, m ? t0 ^ 111 this manner > the motor is disconnected from the line whenever the load is removed. This is called "no-load" protection. ft th if e ,^ P ° int ® tarter can also be used with series motors. In this case the hoiding coil acts as an under-voltage release. If the power line voltage This nr d ? T 'S N V t0 a l0W value ’ the holding coil releases the starter JtartJl 1 thC l n ® VOltage from bein « aPPtied to the motor when the »SniS^p““cU„" m0 '' ed ,r °" the ClrCU “- T “ 8 type d °« »°* pro- « *tUus£atefteToi° r Colmectlons ,or both t »°- “ d three-point starters DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Generator Maintenance Precautions When a generator is installed, it is usually used for a particular job and the installation is permanent. Once the prime mover has been coupled to the generator shaft, the only maintenance work necessary should be oil bearings, etc. If the generator leads are altered, the generator polarity may be reversed or the generator may fail to build up. For example, re- versing the field of a self -excited generator will cancel the residual mag- netism of the field, and the generator will not build up even when the con- nections are corrected. By flashing the field, residual magnetism of the proper polarity can be restored. It should never be necessary to reverse the output polarity of a DC gener- ator. However, if a reversed polarity is desired, the output leads of the generator should be reversed. The field connections should never be re- versed. The field coils are only connected to the terminal board to make their replacement easier, in case they have been damaged. Once the field wires have been properly connected in the initial installation, they should never be changed. 5-68 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Bearings Since DC generators and motors are rotating machines, they depend upon the condition of their bearings for smooth operation. If a bearing is in good condition, the machine will run smoothly. If the bearing is in poor condi- tion, the machine will run poorly, if at all. Generally speaking, there are two types of bearings— friction and anti- friction. The friction bearing, or sleeve bearing, is a soft metal sleeve in which the shaft revolves. The shaft is actually separated from the metal by a thin film of lubricating oil. The shaft therefore rotates on a film of oil, and very little friction results. If there is no lubricating oil, the shaft rubs directly against the surface of the bearing sleeve, and chips of metal gradually accumulate, greatly increasing the friction in the bearing. This can ruin the bearing surfaces and cause the shaft to freeze in the bearing sleeve so that it does not turn at all. If the bearing is properly lubricated, there is no surface contact between the shaft and the surface of the sleeve bearing. As a result, there will be no wear on the bearings as long as they are properly lubricated and the oil is clean. An anti -friction bearing is a ball bearing which uses the rolling action of S n s to eliminate the friction. The balls are enclosed in runways calles races." The space between the balls and the races must be free of dirt or chips which cause the bearing to wear and make it unusable. Ball bearings are packed in grease, which lubricates them and keeps out foreign particles. Some machines sometimes use bearings that do not require lubrication. These bearings, called "self-lubricating" bearings, contain a high per- centage of oil which is forced out of the pores of the metal when the bear- ing becomes heated by rotation. 5-69 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Bearing Lubrication Since bearings are precision-tooled, great care must be exercised in han- dling them, and in the greasing methods and in the type of lubricant used. Improper greasing procedures are a frequent cause of bearing troubles in rotating equipment. An excess of grease in the bearing housing causes the grease to churn around and overheat. This results in a rapid deterioration of the grease and eventual destruction of the bearing. Grease under pres- sure will force its way through the bearing housing seals onto the commu- tator and other motor parts. Grease will eat away insulation and eventually cau$e short circuits and grounds. Most large-sized motors and generators have grease cups which force the grease into the bearings when the cup is turned. It is very important that the right type of lubricant be used on bearings. If the wrong lubricant is used, it can do more harm than good. Therefore, when lubricating a ro- tating machine, always refer to an instruction book on lubrication in order to find out the kind to use. Often the correct lubricant is contained in the spare parts box for the particular machine. 5-70 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Bearing Lubrication (continued) Although excess lubrication causes many troubles and faults in generator or motor operation, the lack of lubrication is also serious. A hear ing which is not properly lubricated will overheat immediately, causing expan- sion of the shaft and bearing assembly. This expansion may be sufficient to stop the shaft rotation. Lack of lubrication also results in noisy opera- tion, due to the direct contact between the shaft and bearing. Bearing housings should be checked periodically for overheating and noisy operation. In normal operation, the temperature of a generator or motor will rise so that the bearing housings normally heat a certain amount. If the housings overheat, do not add or change lubrication without first in- specting the bearing to make certain that lack of lubrication is the cause. Shafts may be forced out of line by a coupling unit or the lubrication may not be reaching all parts of the bearing. 't \ x DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Commutators and Brushes Next to bearings, commutators and brush assemblies are the chief sources of trouble in DC rotating machinery. The continual sliding of the brushes against the commutator wears the brushes down and tends to push them out of alignment, causing trouble in the commutator and brushes. When some- thing does go wrong in commutation, it is accompanied by excessive spark- ing, which aggravates the original trouble and causes additional troubles. For satisfactory commutation of DC machines, a continuous contact must be maintained between commutator and brushes. The commutator must be mechanically true, the unit in good balance, and the brushes in good shape and well adjusted. When correct commutation is taking place, the commutator is a dark chocolate color. This color is due to the action of the brushes in riding on the rotating commutator. The surface of the commutator is smooth. Under normal load there will be very little noticeable sparking. The mica insu- lation between the commutator segments is usually cut below the surface of the segments. The brushes are free to slide up and down in the brush holders and are made to bear upon the commutator with a spring adjusted to produce a pressure of one and a half to 2 pounds per square inch of brush surface. Too little pressure causes poor brush contact and unnec- essary sparking, and too much pressure will cause excessive brush wear. PROPER COMMUTATION Correct pressure Spring Brush 5-72 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Commutators and Brushes (continued) When there is excessive sparking at the commutator and good commutation cannot be obtained, the commutator and brush assembly should be checked and any defect found should be corrected if at all possible. The inspection procedure and the steps taken to eliminate troubles are outlined below: 1. Observe machine under actual operation to see if you can spot anything unusual such as arcing or excessive sparking, which might indicate a loose connection. 2. Turn off the machine, making sure that all power is removed before proceeding with your check. 3. Inspect all connections and make sure that none are loose. 4. Check the relative position of the brushes on the commutator. (They should be on opposite sides of the commutator.) If brushes are un- equally spaced, look for a bent brush holder and eliminate the trouble. 5. Inspect the condition of the brushes. If the brushes are worn badly, they should be replaced. When removing a brush, first lift up the spring lever to release the pressure, then remove brush. Insert new brush, making sure that brush can move freely in the holder. The end of the brush must then be fitted to the commutator by sanding it as illustrated. Adjust the brush spring pressure. Check the pigtail wire and its ter- minal for tightness. The pigtail wire must not touch any metal except the brush holders to which it is attached. 6. Check the commutator for dirt, pitting, irregularities, etc. Dirt can be removed with a piece of light canvas. Fine sandpaper will remove slight roughness. Never use emery cloth on a commutator. 5-73 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Insulation Breakdown Under normal operation, the field and armature winding of generators and motors are completely insulated from the frame of the machine, which is bolted to the deck. A resistance measurement from the frame to the ar- mature or the field should read infinity or several million ohms. Sometimes, due to excessive heat generated by overloading the machine, or because of the high moisture content in the air aboard ship, the high re- sistance of the insulation decreases and some of the current leaks through the insulation to the frame. This leakage current adds to the "breakdown" of the insulation, and if the leakage is not found in time, the breakdown will be complete and the coil will be shorted to the frame. (Such a coil is called a "grounded" coil.) A short circuit will cause the entire winding to overheat and burn out. The armature and field windings should therefore be checked at regular intervals to detect "leaks" and "grounds" before they cause serious damage. An ordinary ohmmeter cannot be used for insulation testing in large prac- tical machines, since the leakage will often show itself only when a high voltage is applied to it. An ohmmeter cannot apply a high enough voltage to test adequately for breakdowns. An instrument called a "megger" is used. The megger supplies the necessary high voltage and is calibrated to read very high resistance values. The illustration below shows how a typical insulation breakdown occurs when the insulation is broken or bruised, or becomes weakened from salt water. Each of the leakage paths shown becomes a low resistance parallel loop through which current flows to ground. 5-74 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING The Megger The megger is an instrument which is used for measuring insulation re- sistance, such as the resistance between windings and the frame of elec- tric machinery, and the insulation resistance of cables, insulators and bushings. The megger consists of two parts — (1) a hand-cranked DC gen- erator (magneto) or a high voltage "B" battery which supplies the voltage for making the measurement and (2) a special type of meter movement. Before using the megger, the circuit is voltage checked to make sure it is de-energized because the megger must only be used on a de-energized cir- cuit. Then both meter leads are connected to ground to ensure a good ground connection and good meter operation. Next, the megger is con- nected across the circuit to be tested and the hand crank is turned, gener- ating a high voltage across the megger terminals. As a result, current flows through the circuit or insulation being tested. This current flow is measured by the meter movement as it is in an ohmmeter, but unlike the ohmmeter, the megger is calibrated to measure megohms. The normal resistance reading for a circuit insulated from ground is several hundred megohms. If the megger reads low, a ground exists, and the shorted cir- cuit should be replaced. A "ground" is a reference point for voltage and resistance measurements in electrical circuits. All large metal objects (such as motor housings, switch boxes, and transformer cases) that are associated with electrical equipment are directly connected to ground. A megger determines if any of the wires inside a motor or transformer have come in contact with the metal housing (have become "grounded") or are in danger of becoming so. 5-75 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING The Megger (continued) Sometimes moisture in the insulation will cause the insulation resistance to be as low as one megohm. Moisture can be eliminated by drying the in- sulation with heaters, lamp banks, or a hot air blower. Field coils can be dried by passing a current through them. To test a DC machine for insulation leakage and grounded 'coils, the leads of the megger are connected between the frame and the external terminals. The crank of the megger must be turned at a steady moderate speed. If the megger reads several megohms or more, the insulation is secure. If the megger reads less than one megohm, some of the insulation is defective somewhere and the leak must be isolated. The field leads must be discon- nected from the armature and both tested separately. The method of testing is shown. To test the field, the megger is connected between one side of the field and the frame. To test the armature, the meg- ger is connected between the shaft and the commutator segments. If the megger reads several megohms, the insulation has its normal allowable leakage resistance. However, if the megger reads lower than this, say less than two megohms, the leakage is excessive and the insulation will eventually break down. Of course, if the megger reads zero, the insulation is broken and the coil is shorted to the frame of the machine. and grounded coil test 5-76 DC MACHINERY MAINTENANCE AND TROUBLESHOOTING Testing Field Coils Hi testing for open and internally shorted field coils, an ohmmeter is used. . ®ld leads are disconnected from the armature to avoid parallel circuits in testing. The ohmmeter is placed across the field leads as shown in the simplified illustration. If the ohmmeter reads infinity, there is an open circuit somewhere in the field winding. The open-circuited coil can be detected bytesting each coil individually. The coil with the open circuit should be disconnected from the other coils and replaced. The armature resistance of a DC machine is normally so low that an ordi- nary ohmmeter will not be able to measure it. The ohmmeter will read practically zero. If the armature has a few shorted turns, the ohmmeter will still read practically zero. If the armature has an open turn, the ohm- meter will also read zero due to the numerous parallel paths. Therefore special equipment is used to test armatures. 5-77 DC MACHINERY Review of DC Generators and Motors Now review what you have learned concerning the basic principles of DC motors and generators. [•TO sKi si A loop of wire rotating in a magnetic field, with slip rings and brushes used to transfer the generated current to an external circuit. tt.T.F.MF.NTARY DC GENERATOR — A loop of wire rotating in a magnetic field, with a commutator and brushes, used to transfer the generated cur- rent to an external circuit. DC MACHINERY Review of DC Generators and Motors (continued) COMMUTATOR SPARKING — Spark- ing at the brushes occurs when the brushes short out the commutator segments of a coil which is generating an emf — a coil not in the neutral plane. Sparking is reduced by shifting the brushes, or by using interpoles or compensating windings. ARMATURE REACTION — The effect of the armature field in shifting the position of the main field. The arma- ture field is caused by current flow in the armature circuit. INTERPOLES — Small poles mounted between the main field windings to generate a field exactly opposite to that of the armature coil and counter- act the effect of armature reaction. SEPARATELY -EXCITED GENERATOR — Generator having a shunt field which is excited by an external source of DC voltage. SELF-EXCITED GENERATORS — Shunt, series and compound generators connected to obtain field excitation from the generator output. Series fields are connected in series with the generator load and use the load cur- rent for field excitation while shunt fields are connected across the gen- erator terminals in parallel with the generator load. 5-79 DC MACHINERY Review of DC Generators and Motors (continued) nr: MOTOR PRINCIPLE — Current flow through the armature coll causes the armature to act like a magnet. The poles of the armature field are attracted to field poles of opposite po- larity, causing the armature to rotate. nr MOTOR COUNTE R-EMF — The rotating armature coil of a DC motor generates a voltage which is opposite in polarity to that of the power line. This generated counter-emf limits the amount of current flow in the ar- mature circuit. DC MOTOR SPEED CON TROL - DC motor speed can be varied by means of a rheostat connected in series with either the armature or field circuit. Increasing the field resistance in- creases speed, while increasing the armature resistance decreases speed. DC MOTOR CHARACTERISTICS — DC shunt, series and compound motor circuit connections are the same as those of the corresponding type of DC generator. Speed and torque versus armature current are used as com- parison characteristics for DC motors. 5-80 DC MACHINERY Review of DC Generators and Motors (continued) DC MOTOR STARTER - A switching circuit containing a resistor connected in series with the armature, to reduce the armature current to a safe value at starting. As the motor speed in- creases, the resistor is shorted out of the circuit and the current is limited by the counter-emf of the armature. TWO -POINT STARTER — DC motor starter having only two connections, one to the DC power line and the other to the motor armature circuit. This type of starter can release automati- cally in case of power line failure, if a holding coil is used. THREE -POINT STARTER — DO motor starter having three terminals —line, armature and field. A holding coil is connected in series with the motor field and releases the starter in case of power line failure or an open field circuit. FOUR-POINT STARTER — DC motor starter having four terminals — two line terminals, field and armature. The holding coil connects directly across the line and the field winding is not in series with this coil. This starter is used when a field rheostat is used for speed control. 5-81 ALTERNATORS Importance of AC Generators A large percentage of the electrical power generated is AC. As a result, the AC generator is the most important means of electrical power produc- tion. AC generators, or "alternators, " vary greatly in size depending up- on their power requirements. For example, the alternators used at hydro- electric plants such as Boulder Dam are tremendous in size, generating hundreds of kilowatts at voltage levels of 13,000 volts. ; Regardless of their size, all electrical generators, whether DC or AC, de- pend upon the action of a coil cutting through a magnetic field or a mag- netic field cutting through a coil. As long as there is relative motion be tween a conductor and a magnetic field, a voltage win be, generated. That part which generates the magnetic field is called the "field, and that part in which the voltage is generated is called the "armature." In order to have relative motion take place between a conductor and a magnetic field, all generators are made up of two mechanical parts— a rotor and a stator. You know that in DC generators the armature is always the rotor. ALTERNATORS Types of Alternators There are two types of alternators — the revolving- armature type and the revolving-field type alternators. The revolving-armature type alternator is similar in construction to the DC generator in that the armature rotates through a stationary magnetic field. In the DC generator, the emf gener- ated in the armature windings is converted into DC by means of the com- mutator, whereas in the alternator, the generated AC is brought to the load unchanged, by means of slip rings. The revolving-armature alternator is found only in alternators of small power rating and is not generally used. The revolving-field type alternator has a stationary armature winding and a rotating field winding. The advantage of having a stationary armature winding is that the generated voltage can be connected directly to the load. A rotating armature would require slip rings to conduct the current from the armature to the outside. Since slip rings are exposed, arc-overs and short circuits result at high generated voltages. Therefore, high-voltage alternators are usually of the rotating field type. The voltage appliedto the rotating field is low DC voltage and, therefore, the problem of arc- over at the slip rings is not encountered. AC Output ROTATING armature ALTERNATOR 4 Field X excitation y ROTATING FIELD A I T E R N ATf ‘ ' A HIT A A \_y X v AIj A i_jiV.il /A 1 V/i V The maximum current that can be supplied by an alternator depends upon the maximum heating loss that can be sustained in the armature. This heating loss (which is an I^R power loss) acts to heat the conductors, and if excessive, to destroy the insulation. Therefore, alternators are rated in terms of this current and in terms of the voltage output — the alternator rating is in volt-amperes, or in more practical units, kilovolt-amperes. 5-83 ALTERNATORS Alternator Construction Alternators having high kilovolt-ampere ratings are of the turbine-driven, high-speed type. The prime mover for this type of alternator is a high- speed steam turbine which is driven by steam under high pressure. Due to the high speed of rotation, the rotor field of the turbine- driven alternator is cylindrical, small in diameter with windings firmly imbedded in slots in its face. The windings are arranged to form two or four distinct poles. Only with this type of construction can the rotor withstand the terrific centrifugal force developed at high speeds without flying apart. In slower speed alternators which are driven by engines, water power, geared turbines and electric motors, a salient-pole rotor is used. In this type of rotor a number of separately wound pole pieces are bolted to the fr am e of the rotor. The field windings are either connected in series or in series groups — connected in parallel. In either case, the ends of the windings connect to slip rings mounted on the rotor shaft. Regardless of the type of rotor field used, its windings are separately excited by a DC generator called an "exciter." The stationary armature or stator of an alternator holds the windings that are cut by the rotating magnetic field. The voltage generated in the ar- mature as a result of this cutting action is the AC power which is applied to the load. The stators of all alternators are essentially the same. The stator con- sists of a laminated iron core with the armature windings embedded in this core. The core is secured to the stator frame. 5-84 ALTERNATORS Single-Phase Alternator A single-phase alternator has all the armature conductors connected in se- ries or parallel; essentially one winding across which an output voltage is generated. If you understand the principle of the single-phase, you will easily understand multi-phase alternator operation. The schematic diagram illustrates a two-pole, single-phase alternator. The stator is two pole because the winding is wound in two distinct pole groups, both poles being wound in the same direction around the stator frame. Observe that the rotor also consists of two pole groups, adjacent poles being of opposite polarity. As the rotor turns, its poles will induce AC voltages in the stator windings. Since one rotor pole is in the same position relative to a stator pole as any other rotor pole, both the stator poles are cut by equal amounts of magnetic lines of force at any time. As a result, the voltages induced in the two poles of the stator winding have the same amplitude or value at a given instant. The two poles of the stator winding are connected to each other so that the AC voltages are in phase, or "series aiding. " Assume that rotor pole 1, a south pole, induces a volt- age with the polarity as shown in stator pole 1. Since rotor pole 2 is a north pole, it will induce the opposite voltage polarity in stator pole 2, in relation to the polarity of the voltage induced in stator pole 1. In order that the voltages in the two poles be series aiding, poles 1 and 2 are con- nected as shown. Observe that the two stator poles are connected in series so that the voltages induced in each pole add to give a total voltage that is two times the voltage in any one pole. 5-85 ALTERNATORS Two- Phase Alternator Multi-phase or polyphase alternators have two or more single-phase windings symmetrically spaced around the stator. In a two-phase alter- nator there are two single-phase windings physically spaced so that the AC voltage induced in one is 90 degrees out of phase with the voltage in- duced in the other. The windings are electrically separate from each other. The only way to get a 90-degree phase difference is to space the two windings so that when one is being cut by maximum flux, the other is being cut by no flux. The schematic diagram illustrates a two-pole, two-phase alternator. The stator consists of two single-phase windings completely separated from each other. Each winding is made up of a series of two windings which are in phase and connected so that their voltages add. The rotor is identical to that used in the single-phase alternator. In the first schematic, the rotor poles are opposite all the windings of phase A. Therefore, the voltage in- duced in phase A is maximum and the voltage induced in phase B is zero. As the rotor continues rotating, it moves away from the A windings and ap- proaches the B windings. As a result, the voltage induced in phase A de- creases from its maximum value and the voltage induced in phase B in- creases from zero. In the second schematic, the rotor poles are opposite the windings of phase B. Now the voltage induced in phase B is maximum, whereas the voltage induced in phase A has dropped to zero. Notice that a 90-degree rotation of the rotor corresponds to one-quarter of a cycle, or 90 degrees. The waveform picture shows the voltages induced in phase A and phase B for one cycle. The two voltages are 90 degrees out of phase. 5-86 ALTERNATORS Two-Phase Alternator (continued) If the phases of a two-phase alternator are connected so that three wires will have to be brought to the outside instead of the original four wires (two for each phase), the alternator is then called a "two-phase, " three- wire alternator, which is illustrated by the schematic. The schematic is simplified in that the rotor is not shown and the entire phase, consisting of a number of windings in series is shown as one winding. The windings are drawn at right angles to each other to represent the 90-degree phase dis- placement between them. The three wires make possible three different load connections, (A) and (B) across each phase, and (C) across both phases. The third voltage is the vector sum of both phase voltages; it is larger in magnitude than either phase voltage and is displaced 45 degrees from either phase. The resultant voltage is equal to the square root of two (| j2^ 1.414) times the phase voltage. 5-87 ALTERNATORS Three-Phase Alternator The three-phase alternator, as the name implies, has three single-phase windings spaced so that the voltage induced in any one is phase-displaced by 120 degrees from the other two. A schematic diagram of a three-phase stator showing all the coils becomes complex, and it is difficult to see what is actually happening. A simplified schematic shows all the windings of a single-phase lumped together as one winding, as illustrated. The rotor is omitted for simplicity. The voltage waveforms generated across each phase are drawn on a graph phase- displaced 120 degrees from each other. The three-phase alternator as shown in this schematic is essentially three single-phase alternators whose generated voltages are out of phase by 120 degrees. The three phases are independent of each other. Rather than have six leads come out of the three-phase alternator, the same leads from each phase are connected together to form a "wye, " or "star, " connection. The point of connection is called the neutral, and the voltage from this point to any one of the line leads will be the phase volt- age. The total voltage or line voltage, across any two line leads is the vector sum of the individual phase voltages. The line voltage is 1. 73 times the phase voltage. Since the windings form only one path for current flow between phases, the line and phase currents are equal. A three-phase stator can also be connected so that the phases are connected end-to-end; it is now called "delta connected. " In the delta connection the line voltages are equal to the phase voltage, but the line currents will be equal to the vector sum of the phase currents. Since the phases are 120 de- grees out of phase, the line current will be 1. 73 times the phase current. Both the "wye" and the "delta" connections are used for alternators. 5-88 ALTERNATORS Frequency and Voltage Regulation The frequency of the AC generated by an alternator depends upon the num- ber of poles and the speed of the rotor. When a rotor has rotated through an angle so that two adjacent rotor poles (a north and a south) have passed one winding, the voltage induced in that one winding will have varied through a complete cycle of 360 electrical degrees. The more poles there are, the lower the speed of rotation will be for a given frequency. A two- pole machine must rotate at twice the speed of a four-pole machine to gen- erate the same frequency. The magnitude of the voltage generated by an alternator is varied by vary- ing the field strength (field current). In an alternator, just as in a DC generator, the output voltage varies with the load. In addition to the IR drop, there is another voltage drop in the windings called the IXl drop. The IXl drop is due to the inductive react- ance of the windings. Both the IR drop and the IX^ drop decrease the out- put voltage as the load increases. The change in voltage from no-load to full-load is called the voltage regulation of an alternator. A constant volt- age output from an alternator is maintained by varying the field strength as required by changes in load. 5-89 ALTERNATORS Three -Phase Connections The majority of all alternators in use today are three-phase winding ma- chines. This is because three-phase alternators are much more efficient than either two-phase or single-phase alternators. The stator coils of three-phase alternators may be joined together in either "wye" or "delta" connections as shown below. With this type of connection only three wires come out of the alternator, and this allows convenient connection to other three-phase equipment. It is common to use three- phase transformers in connection with this type of system. Such a device may be made up of three single-phase transformers connected in the same way as for alternators. If both the primary and secondary are connected in wye, the transformer is called "wye-wye. " If both windings are con- nected in delta, the transformer is called a "delta-delta. " Delta connected Wye connected ALTERNATORS Review AC GENERATORS — An AC generator is essentially a loop rotating through a magnetic field. The cutting action of the loop through the magnetic field gen- erates AC in the loop. This AC is re- moved from the loop by means of slip rings and applied to an external load. HELDS - The armature ia stationary anniiin Hi H « h voltages can be generated in the armature and i n he l °^ dir ,! ct i y without the need 01 S»P rings and brushes. The S« n^° ^ ^ f t0 r0t0r field by means ° f slip rings, but this does not introduce any insulation problems. ’ SINGLE-PHASE ALTERNATOR — A single-phase alternator has an arma- ture which consists of a number of windings placed symmetrically around the stator and connected in series. The voltages generated in each winding add to produce the total voltage across the two output terminals. TWO-PHASE ALTERNATOR - The two-phase alternator consists of two phases whose windings are so placed around the stator that the voltages gen- erated in them are 90 degrees out of phase. THREE-PHASE ALTERNATOR — in the three-phase alternator, the wind- ings have voltages generated in them which are 120 degrees out of phase. Three-phase alternators are most of- ten used to generate AC power. ALTERNATOR FREQUENCY — The frequency of the AC generated by an alternator depends upon the speed erf rotation and the number of pairs of rotor poles. The voltage regulation of an alternator is poorer than that of a DC generator because of the IXl drop in the armature winding. 5-91 ALTERNATING CURRENT MOTORS Types of AC Motors Since a major part of all electrical power generated is AC, many motors are designed for AC operation. AC motors can, in most cases, duplicate the operation of DC motors and are less troublesome to operate. This is because DC machines encounter difficulties due to the action of commuta- tion which involves brushes, brush holders, neutral planes, etc. Many types of AC motors do not even use slip rings, with the result that they give trouble-free operation over long periods of time. AC motors are particularly well suited for constant speed applications, since the speed is determined by the frequency of the AC applied to the motor terminals. AC motors are also made that have variable speed characteristics within certain limits. AC motors can be designed to operate from a single -phase AC line or a multi-phase AC line. Whether the motor is single-phase or multi-phase, it operates on the same principle. This principle is that the AC applied to the motor generates a rotating magnetic field, and it is this rotating mag- netic field that causes the rotor of the motor to turn. AC motors are generally classified into two types: (1) the synchronous motor and (2) the induction motor. The synchronous motor is an alternator operated as a motor, in which three-phase AC is applied to the stator and DC is applied to the rotor. The induction motor differs from the syn- chronous motor in that it does not have its rotor connected to any source of power. Of the two types of AC motors mentioned, the induction motor is by far the most commonly used. AC MOTORS 5-92 ALTERNATING CURRENT MOTORS Rotating Field Before learning how a rotating magnetic field will cause an energized rotor to turn, the thing for you to find out is how a rotating magnetic field can be Iiroduced. The schematic illustrates a three-phase rotor to which three - f .f- 6 j applied from a three phase source like the alternator you studied. The windings are connected in delta as shown. The two windings in each phase are wound in the same direction. At any instant the magnetic field generated by one particular phase depends upon the current through that phase. If the current is zero, the magnetic field is zero. If the cur- 18 * ma * lmum » the magnetic field is a maximum. Since the currents w , md “g s a re 120 degrees out of phase, the magnetic fields gen- *™t ed wi 11 also be 120 degrees out of phase. Now, the three magnetic fields that exist at any instant will combine to produce one field, which acts * up ° n f°tor. You will see on the following page that from one instant to the next, the magnetic fields combine to produce a magnetic field whose through a certain angle. At the end of one cycle of AC, the magnetic field will have shifted through 360 degrees, or one revolution. 5-93 The drawing shows the three current waveforms applied *to the stator. These waveforms are 120 degrees out of phase with each other. The wave- forms can represent either the three alternating magnetic fields generated by the three phases or the currents in the phases. The waveforms are lettered to correspond to their associated phase. Using the waveforms, we can combine the magnetic fields generated every 1/6 of a cycle (60 cycles) to determine the direction of the resultant magnetic field. At point 1, waveform C is positive and waveform B is negative. This means that the current flows in opposite directions through phases B and C. This estab- lishes the magnetic polarity of phases B and C. The polarity is shown on the simplified diagram above point 1. Observe that Bj is a north pole and B is a south pole, and C is a north pole while Ci is a south pole. Since at point 1 there is no current flowing through phase A, its magnetic field is zero. The magnetic fields leaving poles Bj and C will move toward the nearest south poles Ci and B. as shown. Since the magnetic fields of Band C are equal in amplitude, the resultant magnetic field will lie between the two fields and will have the direction as shown. At point 2,60 degrees later, the input current waveforms to phases A and B are equal and opposite, and waveform C is zero. 'You can see that now the resultant magnetic-field has rotated through 60 degrees. At Point 3, wave- form B is zero and the resultant magnetic field has rotated through another 60 degrees. From points 1 through 7 (corresponding to one cycle of AC), you can see that the resultant magnetic field rotates through one revolution for every cycle of AC supplied to the stator. The conclusion is that, by applying three-phase AC to three windings sym- metrically spaced around a stator, a rotating magnetic field is generated. 5-94 ALTERNATING CURRENT MOTORS Synchronous Motor The construction of the synchronous motor is essentially the same as the construction of the salient-pole alternator. In order to understand how the synchronous motor works, assume for the moment that the application of three-phase AC to the stator causes a rotating magnetic field to be set up around the rotor. Since the rotor is energized with DC, it acts like a bar a bar ma S net is Pivoted within a magnetic field, it will turn until it lines up with the magnetic field. If the magnetic field turns, the rax magnet will turn with the field. If the rotating magnetic field is strong, it will exert a strong turning force on the bar magnet. The magn et will therefore be able to turn a load as it rotates in step with the rotating mag- netic field. & THE ROTOR r %, WITH THE MAGNETIC FIELD Advantages of Synchronous Motors 1. Used for constant speed 2. U.ed for ^teto^ection by oyer-e*citing rotor Hold ALTERNATING CURRENT MOTORS Synchronous Motor (continued) One of the disadvantages of a synchronous motoris that it cannot from a standstill by applying three-phase AC to the stator. The instont AC is applied to the stator, a high-speed rotating field appears immedmtely^ This rotating field rushes past the rotor poles so quickly the rotor does not have a chance to get started. The instant AC is applied to the stator of a synchronous motor, a high speed rotating magnetic field appears immediately. This rotating magnetic field rushes past the rotor poles so quickly that the rotor is repelled first in one direction and then the other. A synchronous motor in its pure form has no starting torque. Generally synchronous motors are started as squirrel cage motors; a squirrel cage winding is placed on the rotor as shown. To start the motor, the stator is energized and the DC supply to the field is not energized. The squirrel cage winding brings the rotor up to near syn- chronous speed; then the DC field is energized, locking the rotor in step with the rotating magnetic field. Synchronous motors are used for loads that require constant speed from no-load to full-load. 5-96 ALTERNATING CURRENT MOTORS Induction Motors The induction motor is the most commonly used AC motor because of its simplicity, its rugged construction and its low manufactur ing cos t. These characteristics of the induction motor are due to the fact that the rotor is a self-contained unit which is not connected to the external source of volt- age. The induction motor derives its name from the fact that AC currents are induced in the rotor circuit by the rotating magnetic field in the stator. The stator construction of the induction motor and the synchronous motor are almost identical but their rotors are completely different. The induc- tion rotor is made of a laminated cylinder with slots in its surface. The windings in these slots are one of two types. The most common is called a squirrel-cage winding." This winding is made up of heavy copper bars connected together at each end by a metal ring made of copper or brass. No insulation is required between the core and the bars because of the very low voltages generated in the rotor bars. The air gap between the rotor and stator is very small to obtain maximum field strength. The other type of winding contains coils placed in the rotor slots. The ro- tor is then called a "wound rotor." Regardless of the type of rotor used, the basic principle of operation is the same. The rotating magnetic field generated in the stator induces a magnetic field in the rotor. The two fields interact and cause the rotor to turn. 5-97 ALTERNATING CURRENT MOTORS Induction Motors— How They Work When AC is applied to the stator windings, a rotating magnetic field is gen- erated. This rotating field cuts the bars of the rotor and induces a current in them. As you know from your study of meter movements and elementary motors, this induced current will generate a magnetic field around the con- ductors of the rotor which will try to line up with the stator field. How- ever, since the stator field is rotating continuously, the rotor cannot line up with it but must always follow along behind it. o* 1 Current flow induced in the rotor Stator Pole Rotor Field due to induced current ’ \ in rotor As you know from Lenz's Law, any induced current --tries to oppose the changing field which induces it. In the case of an induction motor, the change is the motion of the resultant stator field, and the force exerted on the rotor by induced current and field in the rotor is such as to try to cancel out the continuous motion of stator field— that Is. the rotor will move in the same direction, as close to the moving stator fiela as its weight and its load will allow it. ALTERNATING CURRENT MOTORS Induction Motors— Slip tt is impossible for the squirrel-cage rotor of an induction motor to turn at the same speed as the rotating magnetic field. If the speeds were the same, no relative motion would exist between the two and no induced emf would result in the rotor. Without induced emf, a turning force would not be exerted on the rotor. The rotor must rotate at a speed less than that of the rotating magnetic field, if relative motion is to exist between the two The percentage difference between the speed of the rotating stator field and the rotor speed is called "slip." The smaller the slip, the closer the rotor speed approaches the stator field speed. Field and Rotor Turning at the Same Speed At the same speed the field lines do not move across this conductor of the rotor No emf induced in the rotor SLIP — Rotor Turning Slower than the Field Rotor slip results in field cutting across the rotor conductor ROTOR SLIP 100 % Emf induced because stator speed is greater than rotor speed S s = Synchronous Speed Rg = Rotor Speed The bw e d r°tU h f d T nds u P° n the torque requirements of the load, rotor lhe turnES? f 8tr0nger the turning force needed to rotate the creases^and^ttis^Pmf 0 ^ 6 ?“ lncrease onl y if the rotor induced emf in- the rotor afo / f increase onl y « the magnetic field cuts through the rotor at a faster rate. To increase the relative speed between the field and rotor, he rotor must slow down. Therefore for heavieTloads the in- sHp-ht rh m 0r Wl11 tU ? Sl0wer than for u g h ter loads. Actually only a slight change m speed is necessary to produce the usual current Changes ha^su^' f n ° rn ? al Changes in load - This is because the rotor windings 48 a res “ 11 ' »*» ™ ^ 5-99 ALTERNATING CURRENT MOTORS Two Phase Induction Motors Induction motors are designed for three-phase,two-phase, or single-phase operation. In each case the AC applied to the stator must generate a ro- tating field which will pull the rotor with it. You have already seen how three-phase AC, applied to a three-phase symmetrically distributed winding, will generate a rotating magnetic field. A two-phase induction motor has its stator made up of two windings which are placed at right angles to each other around the stator. The simplified drawing illustrates a two-phase stator. The other drawing is a schematic of a two-phase induction motor. The dotted circle represents the short- circuited rotor winding. If the voltages applied to phases A-Aj and*B-Bi are 90 degrees out of phase, the currents that will flow in the phases will be displaced by 90 de- grees. Since the magnetic fields generated in the coils will be in phase with their respective currents, the magnetic fields will also be 90 degrees out of phase with each other. These two out-of-phase magnetic fields, whose coil axes are at right angles to each other will add together at every instant during their cycle to produce a resultant field which will rotate one revolution for each cycle of AC. 5-100 ALTERNATING CURRENT MOTORS Two Phase Induction Motors (continued) Jkf a gra . ph ° f the alternating magnetic fields which are displaced 90 degrees m phase. The waveforms are lettered to corre- r nd ,° th ? lr ass ° ciated Phase. At position 1, the current flow and mag- netic field in winding A-Aj is a maximum and the current flow and mag- netic field m winding B-Bj is zero. The resultant magnetic field will therefore be m the direction of the winding A-A x axis. At the 45 degree pomt (position 2), the resultant magnetic field will lie midway between windings ^ -A 1 and B-Bi, smce the coil currents and magnetic fields are eqiml in strength. At 90 degrees (position 3), the magnetic field in winding A-Ai is zero and the magnetic field in winding B-B! is a maximum. Now the resultant magnetic field lies along the axis of the B-Bi winding as shown. The resultant magnetic field has rotated through 90 degrees to get from position 1 to position 3. 6 At . 135 'degrees (position 4), the magnetic fields are again equal in ampli- tude. However, the magnetic field in winding A-Ai has reversed its direc- tion. The resultant magnetic field, therefore, lies midway between the windings and points in the direction as shown. At 180 degrees (position 5), he magnetic field is zero in winding B-Bj and a maximum in winding A : A 1- T A he 4 resultant magnetic field will, therefore, lie along the axis of winding A-Aj as shown. From 180 degrees to 360 degrees (positions 5 to 9), the resultant magnetic Held rotates through another half-cycle and completes a revolution. Thus by placing two windings at right angles to each other and by exciting these windings with voltages 90 degrees out of phase, a Rotating magnetic field will result. “Vtafnetcc *?celd 5-101 ALTERNATING CURRENT MOTORS Single -Phase Motors A single -phase induction motor has only one phase and runs on single -phase AC. This motor finds extensive use in applications which require small low- output motors. The advantage to using single -phase motors is that in small sizes they are less expensive to manufacture than other motor types. Also they eliminate the need for three-phase AC lines. Single -phase motors are used in interior communication equipment, fans, refrigerators, portable drills, grinders, etc. Single-phase motors are divided into two groups: (1) induction motors and (2) series motors. Induction motors use the squirrel cage rotor and a suitable starting device. Series motors resemble DC machines because they have commutators and brushes. SINGLE-PHASE MOTORS ALTERNATING CURRENT MOTORS Single-Phase Induction Motors A single-phase induction motor has only one stator winding. This winding generates a field which alternates along the axis of the single winding, rather than rotating. If the rotor is stationary, the expanding and collapsing stator field induces currents in the rotor. These currents generate a rotor field exactly opposite in polarity to that of the stator. The opposition of the fields exerts a turning force on the upper and lower parts of the rotor trying to turn it 180 degrees from its position. Since these forces are ex- erted through the center of the rotor, the turning force is equal in each di- rection. As a result, the rotor does not turn. However, if the rotor is started turning it will continue to rotate in the direction in which it is started, since the turning force in that direction is. aided by the momentum of the rotor. The rotor will increase speed until it turns nearly 180 degrees for each alternation of the stator field. Since slip is necessary to cause an in- duced rotor current, at maximum speed the rotor turns slightly less than 180 degrees each time the stator field reverses polarity. ALTERNATING CURRENT MOTORS Split-Phase Induction Motors — Capacitor -Start You have seen that once the single-phase motor is started turning it will continue to rotate by itself. It is impractical to start a motor by turning it over by hand, and so an electric device must be incorporated into the stator circuit which will cause a rotating field to be generated upon starting. Once the motor has started, this device can be switched out of the stator as the rotor and stator together will generate their own rotating field to keep the motor turning. One type of induction motor which incorporates a starting device is called a "split-phase induction motor." This motor uses combinations of induct- ance, capacitance and resistance to develop a rotating field. The first type of split-phase induction motor that you will learn about is the capacitor-start type. The diagram shows a simplified schematic of a typical capacitor-start motor. The stator consists of the main winding and a starting winding which is connected in parallel with the main winding and spaced at right angles to it. The 90-degree electrical phase difference be- tween the two windings is obtained by connecting the auxiliary winding in series with a capacitor and starting switch. Upon starting, the switch is closed, placing the capacitor in series with the auxiliary winding. The ca- pacitor is of such a value that the auxiliary winding is effectively a resistive-capacitive circuit in which the current leads the line voltage by approximately 45 degrees. The main winding has enough resistance to cause the current to lag the line voltage by approximately 45 degrees. The two currents are therefore 90 degrees out of phase and so are the magnetic fields which they generate. The effect is that the two windings act like a two-phase stator and produce the revolving field required to start the motor CAPACITOR START 1 SPLIT PHASE | INDUCTION MOTOR When nearly full speed is obtained, a device cuts out the starting winding and the motor runs as a plain single-phase induction motor. Since the special starting winding is only a light winding, the motor does not develop sufficient torque to start heavy loads. Split-phase motors, therefore, come only in small sizes. Since a two-phase in- duction motor is more efficient than a single-phase motor, it is often desirable to keep the auxiliary winding permanently in the circuit so that the motor will run as a two-phase Induction motor. The starting capacitor is usually quite large in order to allow a large current to flow through the auxiliary winding. The motor can thus build up a large starting torque. When the motor comes up to speed, it is not necessary that the auxiliary winding continue to draw the full starting current, and the capacitor can be reduced. Therefore two condensers are used in parallel for starting, and one is cut out when the motor comes up to speed. Such a motor is called a "capacitor -start, capacitor-run induction motor." r Main Winding AC S Ingle - Phase Supply Single- Starting g Phase Winding jg Supply